Redundant field read in Vec::clear() #51802
Labels
A-LLVM
Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.
C-enhancement
Category: An issue proposing an enhancement or a PR with one.
I-slow
Issue: Problems and improvements with respect to performance of generated code.
WG-llvm
Working group: LLVM backend code generation
Comments
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On while 0 < len {
len -= 1;
}Maybe we should mark |
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@newpavlov No, it's not that, and the function is clearly inlined. I think I'm not sure it's correct (there might be an off-by-one, and I'm not sure if there's any issue with EDIT: nope, my proposal doesn't work, because it needs to handle panics. |
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Hm, the only difference is that in |
Yeah, well, that's what I hoped for, but This might need to be fixed in LLVM. |
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Maybe some kind of |
Mark-Simulacrum
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Aug 1, 2018
…chton Use SetLenOnDrop in Vec::truncate() This avoids a redundant length check in some cases when calling `Vec::truncate` or `Vec::clear`. Fixes rust-lang#51802 Note that the generated code still seems suboptimal. I tested with the following functions: ```rust #[no_mangle] pub extern fn foo(x: &mut Vec<u8>) { x.clear(); } #[no_mangle] pub extern fn bar(x: &mut Vec<u8>) { x.truncate(5); } #[no_mangle] pub extern fn baz(x: &mut Vec<u8>, n: usize) { x.truncate(n); } #[no_mangle] pub extern fn foo_string(x: &mut Vec<String>) { x.clear(); } #[no_mangle] pub extern fn bar_string(x: &mut Vec<String>) { x.truncate(5); } #[no_mangle] pub extern fn baz_string(x: &mut Vec<String>, n: usize) { x.truncate(n); } ``` <details> <summary>Old output</summary> ```asm 00000000000460a0 <foo>: 460a0: 48 83 7f 10 00 cmpq $0x0,0x10(%rdi) 460a5: 74 08 je 460af <foo+0xf> 460a7: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi) 460ae: 00 460af: c3 retq 00000000000460b0 <bar>: 460b0: 48 83 7f 10 06 cmpq $0x6,0x10(%rdi) 460b5: 72 08 jb 460bf <bar+0xf> 460b7: 48 c7 47 10 05 00 00 movq $0x5,0x10(%rdi) 460be: 00 460bf: c3 retq 00000000000460c0 <baz>: 460c0: 48 39 77 10 cmp %rsi,0x10(%rdi) 460c4: 76 04 jbe 460ca <baz+0xa> 460c6: 48 89 77 10 mov %rsi,0x10(%rdi) 460ca: c3 retq 460cb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 00000000000460d0 <foo_string>: 460d0: 41 57 push %r15 460d2: 41 56 push %r14 460d4: 53 push %rbx 460d5: 48 8b 47 10 mov 0x10(%rdi),%rax 460d9: 48 85 c0 test %rax,%rax 460dc: 74 4a je 46128 <foo_string+0x58> 460de: 49 89 fe mov %rdi,%r14 460e1: 48 8b 0f mov (%rdi),%rcx 460e4: 48 8d 14 40 lea (%rax,%rax,2),%rdx 460e8: 48 8d 58 ff lea -0x1(%rax),%rbx 460ec: 4c 8d 3c d1 lea (%rcx,%rdx,8),%r15 460f0: 49 83 c7 f0 add $0xfffffffffffffff0,%r15 460f4: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 460fb: 00 00 00 460fe: 66 90 xchg %ax,%ax 46100: 49 89 5e 10 mov %rbx,0x10(%r14) 46104: 49 8b 37 mov (%r15),%rsi 46107: 48 85 f6 test %rsi,%rsi 4610a: 74 0e je 4611a <foo_string+0x4a> 4610c: 49 8b 7f f8 mov -0x8(%r15),%rdi 46110: ba 01 00 00 00 mov $0x1,%edx 46115: e8 a6 e9 ff ff callq 44ac0 <__rust_dealloc@plt> 4611a: 48 83 c3 ff add $0xffffffffffffffff,%rbx 4611e: 49 83 c7 e8 add $0xffffffffffffffe8,%r15 46122: 48 83 fb ff cmp $0xffffffffffffffff,%rbx 46126: 75 d8 jne 46100 <foo_string+0x30> 46128: 5b pop %rbx 46129: 41 5e pop %r14 4612b: 41 5f pop %r15 4612d: c3 retq 4612e: 66 90 xchg %ax,%ax 0000000000046130 <bar_string>: 46130: 41 57 push %r15 46132: 41 56 push %r14 46134: 53 push %rbx 46135: 4c 8b 7f 10 mov 0x10(%rdi),%r15 46139: 49 83 ff 06 cmp $0x6,%r15 4613d: 72 49 jb 46188 <bar_string+0x58> 4613f: 49 89 fe mov %rdi,%r14 46142: 48 8b 07 mov (%rdi),%rax 46145: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 46149: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 4614d: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 46151: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 46158: 00 00 00 4615b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 46160: 49 83 c7 ff add $0xffffffffffffffff,%r15 46164: 4d 89 7e 10 mov %r15,0x10(%r14) 46168: 48 8b 33 mov (%rbx),%rsi 4616b: 48 85 f6 test %rsi,%rsi 4616e: 74 0e je 4617e <bar_string+0x4e> 46170: 48 8b 7b f8 mov -0x8(%rbx),%rdi 46174: ba 01 00 00 00 mov $0x1,%edx 46179: e8 42 e9 ff ff callq 44ac0 <__rust_dealloc@plt> 4617e: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 46182: 49 83 ff 05 cmp $0x5,%r15 46186: 77 d8 ja 46160 <bar_string+0x30> 46188: 5b pop %rbx 46189: 41 5e pop %r14 4618b: 41 5f pop %r15 4618d: c3 retq 4618e: 66 90 xchg %ax,%ax 0000000000046190 <baz_string>: 46190: 41 57 push %r15 46192: 41 56 push %r14 46194: 41 54 push %r12 46196: 53 push %rbx 46197: 50 push %rax 46198: 4c 8b 67 10 mov 0x10(%rdi),%r12 4619c: 49 39 f4 cmp %rsi,%r12 4619f: 76 46 jbe 461e7 <baz_string+0x57> 461a1: 49 89 f6 mov %rsi,%r14 461a4: 49 89 ff mov %rdi,%r15 461a7: 48 8b 07 mov (%rdi),%rax 461aa: 4b 8d 0c 64 lea (%r12,%r12,2),%rcx 461ae: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 461b2: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 461b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 461bd: 00 00 00 461c0: 49 83 c4 ff add $0xffffffffffffffff,%r12 461c4: 4d 89 67 10 mov %r12,0x10(%r15) 461c8: 48 8b 33 mov (%rbx),%rsi 461cb: 48 85 f6 test %rsi,%rsi 461ce: 74 0e je 461de <baz_string+0x4e> 461d0: 48 8b 7b f8 mov -0x8(%rbx),%rdi 461d4: ba 01 00 00 00 mov $0x1,%edx 461d9: e8 e2 e8 ff ff callq 44ac0 <__rust_dealloc@plt> 461de: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 461e2: 4d 39 f4 cmp %r14,%r12 461e5: 77 d9 ja 461c0 <baz_string+0x30> 461e7: 48 83 c4 08 add $0x8,%rsp 461eb: 5b pop %rbx 461ec: 41 5c pop %r12 461ee: 41 5e pop %r14 461f0: 41 5f pop %r15 461f2: c3 retq 461f3: 90 nop 461f4: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 461fb: 00 00 00 461fe: 66 90 xchg %ax,%ax ``` </details> <details> <summary>New output</summary> ```asm 0000000000084d10 <foo>: 84d10: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi) 84d17: 00 84d18: c3 retq 84d19: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 0000000000084d20 <bar>: 84d20: 48 8b 47 10 mov 0x10(%rdi),%rax 84d24: 48 83 f8 05 cmp $0x5,%rax 84d28: b9 05 00 00 00 mov $0x5,%ecx 84d2d: 48 0f 42 c8 cmovb %rax,%rcx 84d31: 48 89 4f 10 mov %rcx,0x10(%rdi) 84d35: c3 retq 84d36: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84d3d: 00 00 00 0000000000084d40 <baz>: 84d40: 48 8b 47 10 mov 0x10(%rdi),%rax 84d44: 48 39 f0 cmp %rsi,%rax 84d47: 48 0f 47 c6 cmova %rsi,%rax 84d4b: 48 89 47 10 mov %rax,0x10(%rdi) 84d4f: c3 retq 0000000000084d50 <foo_string>: 84d50: 41 57 push %r15 84d52: 41 56 push %r14 84d54: 53 push %rbx 84d55: 49 89 fe mov %rdi,%r14 84d58: 4c 8b 7f 10 mov 0x10(%rdi),%r15 84d5c: 4d 85 ff test %r15,%r15 84d5f: 74 2f je 84d90 <foo_string+0x40> 84d61: 49 8b 06 mov (%r14),%rax 84d64: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 84d68: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 84d6c: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84d70: 48 8b 33 mov (%rbx),%rsi 84d73: 48 85 f6 test %rsi,%rsi 84d76: 74 0e je 84d86 <foo_string+0x36> 84d78: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84d7c: ba 01 00 00 00 mov $0x1,%edx 84d81: e8 1a b1 ff ff callq 7fea0 <__rust_dealloc@plt> 84d86: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84d8a: 49 83 c7 ff add $0xffffffffffffffff,%r15 84d8e: 75 e0 jne 84d70 <foo_string+0x20> 84d90: 49 c7 46 10 00 00 00 movq $0x0,0x10(%r14) 84d97: 00 84d98: 5b pop %rbx 84d99: 41 5e pop %r14 84d9b: 41 5f pop %r15 84d9d: c3 retq 84d9e: 66 90 xchg %ax,%ax 0000000000084da0 <bar_string>: 84da0: 41 57 push %r15 84da2: 41 56 push %r14 84da4: 53 push %rbx 84da5: 49 89 fe mov %rdi,%r14 84da8: 4c 8b 7f 10 mov 0x10(%rdi),%r15 84dac: 49 83 ff 06 cmp $0x6,%r15 84db0: 72 44 jb 84df6 <bar_string+0x56> 84db2: 49 8b 06 mov (%r14),%rax 84db5: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 84db9: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 84dbd: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84dc1: 49 83 c7 fb add $0xfffffffffffffffb,%r15 84dc5: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84dcc: 00 00 00 84dcf: 90 nop 84dd0: 48 8b 33 mov (%rbx),%rsi 84dd3: 48 85 f6 test %rsi,%rsi 84dd6: 74 0e je 84de6 <bar_string+0x46> 84dd8: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84ddc: ba 01 00 00 00 mov $0x1,%edx 84de1: e8 ba b0 ff ff callq 7fea0 <__rust_dealloc@plt> 84de6: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84dea: 49 83 c7 ff add $0xffffffffffffffff,%r15 84dee: 75 e0 jne 84dd0 <bar_string+0x30> 84df0: 41 bf 05 00 00 00 mov $0x5,%r15d 84df6: 4d 89 7e 10 mov %r15,0x10(%r14) 84dfa: 5b pop %rbx 84dfb: 41 5e pop %r14 84dfd: 41 5f pop %r15 84dff: c3 retq 0000000000084e00 <baz_string>: 84e00: 41 57 push %r15 84e02: 41 56 push %r14 84e04: 41 54 push %r12 84e06: 53 push %rbx 84e07: 50 push %rax 84e08: 49 89 ff mov %rdi,%r15 84e0b: 48 8b 47 10 mov 0x10(%rdi),%rax 84e0f: 49 89 c4 mov %rax,%r12 84e12: 49 29 f4 sub %rsi,%r12 84e15: 76 3c jbe 84e53 <baz_string+0x53> 84e17: 49 89 f6 mov %rsi,%r14 84e1a: 49 8b 0f mov (%r15),%rcx 84e1d: 48 8d 04 40 lea (%rax,%rax,2),%rax 84e21: 48 8d 1c c1 lea (%rcx,%rax,8),%rbx 84e25: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84e29: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 84e30: 48 8b 33 mov (%rbx),%rsi 84e33: 48 85 f6 test %rsi,%rsi 84e36: 74 0e je 84e46 <baz_string+0x46> 84e38: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84e3c: ba 01 00 00 00 mov $0x1,%edx 84e41: e8 5a b0 ff ff callq 7fea0 <__rust_dealloc@plt> 84e46: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84e4a: 49 83 c4 ff add $0xffffffffffffffff,%r12 84e4e: 75 e0 jne 84e30 <baz_string+0x30> 84e50: 4c 89 f0 mov %r14,%rax 84e53: 49 89 47 10 mov %rax,0x10(%r15) 84e57: 48 83 c4 08 add $0x8,%rsp 84e5b: 5b pop %rbx 84e5c: 41 5c pop %r12 84e5e: 41 5e pop %r14 84e60: 41 5f pop %r15 84e62: c3 retq 84e63: 90 nop 84e64: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84e6b: 00 00 00 84e6e: 66 90 xchg %ax,%ax ``` </details> For calling `truncate` with non-zero lengths on non-`Drop` types, it seems that a redundant load and comparison gets replaced with an awkward sequence with a conditional move. In the unknown length case, the new code is no longer awkward. Maybe someone moderately proficient at assembly could tell if this looks like a win or not. --- This came up when discussing replacing `unsafe { vec.set_len(0) }` with `vec.clear()` in a project where the author was worried about potential performance degradation. It might be worth replacing some unsafe code, even it it's trivial to see that it's actually safe.
Mark-Simulacrum
added a commit
to Mark-Simulacrum/rust
that referenced
this issue
Aug 1, 2018
…chton Use SetLenOnDrop in Vec::truncate() This avoids a redundant length check in some cases when calling `Vec::truncate` or `Vec::clear`. Fixes rust-lang#51802 Note that the generated code still seems suboptimal. I tested with the following functions: ```rust #[no_mangle] pub extern fn foo(x: &mut Vec<u8>) { x.clear(); } #[no_mangle] pub extern fn bar(x: &mut Vec<u8>) { x.truncate(5); } #[no_mangle] pub extern fn baz(x: &mut Vec<u8>, n: usize) { x.truncate(n); } #[no_mangle] pub extern fn foo_string(x: &mut Vec<String>) { x.clear(); } #[no_mangle] pub extern fn bar_string(x: &mut Vec<String>) { x.truncate(5); } #[no_mangle] pub extern fn baz_string(x: &mut Vec<String>, n: usize) { x.truncate(n); } ``` <details> <summary>Old output</summary> ```asm 00000000000460a0 <foo>: 460a0: 48 83 7f 10 00 cmpq $0x0,0x10(%rdi) 460a5: 74 08 je 460af <foo+0xf> 460a7: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi) 460ae: 00 460af: c3 retq 00000000000460b0 <bar>: 460b0: 48 83 7f 10 06 cmpq $0x6,0x10(%rdi) 460b5: 72 08 jb 460bf <bar+0xf> 460b7: 48 c7 47 10 05 00 00 movq $0x5,0x10(%rdi) 460be: 00 460bf: c3 retq 00000000000460c0 <baz>: 460c0: 48 39 77 10 cmp %rsi,0x10(%rdi) 460c4: 76 04 jbe 460ca <baz+0xa> 460c6: 48 89 77 10 mov %rsi,0x10(%rdi) 460ca: c3 retq 460cb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 00000000000460d0 <foo_string>: 460d0: 41 57 push %r15 460d2: 41 56 push %r14 460d4: 53 push %rbx 460d5: 48 8b 47 10 mov 0x10(%rdi),%rax 460d9: 48 85 c0 test %rax,%rax 460dc: 74 4a je 46128 <foo_string+0x58> 460de: 49 89 fe mov %rdi,%r14 460e1: 48 8b 0f mov (%rdi),%rcx 460e4: 48 8d 14 40 lea (%rax,%rax,2),%rdx 460e8: 48 8d 58 ff lea -0x1(%rax),%rbx 460ec: 4c 8d 3c d1 lea (%rcx,%rdx,8),%r15 460f0: 49 83 c7 f0 add $0xfffffffffffffff0,%r15 460f4: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 460fb: 00 00 00 460fe: 66 90 xchg %ax,%ax 46100: 49 89 5e 10 mov %rbx,0x10(%r14) 46104: 49 8b 37 mov (%r15),%rsi 46107: 48 85 f6 test %rsi,%rsi 4610a: 74 0e je 4611a <foo_string+0x4a> 4610c: 49 8b 7f f8 mov -0x8(%r15),%rdi 46110: ba 01 00 00 00 mov $0x1,%edx 46115: e8 a6 e9 ff ff callq 44ac0 <__rust_dealloc@plt> 4611a: 48 83 c3 ff add $0xffffffffffffffff,%rbx 4611e: 49 83 c7 e8 add $0xffffffffffffffe8,%r15 46122: 48 83 fb ff cmp $0xffffffffffffffff,%rbx 46126: 75 d8 jne 46100 <foo_string+0x30> 46128: 5b pop %rbx 46129: 41 5e pop %r14 4612b: 41 5f pop %r15 4612d: c3 retq 4612e: 66 90 xchg %ax,%ax 0000000000046130 <bar_string>: 46130: 41 57 push %r15 46132: 41 56 push %r14 46134: 53 push %rbx 46135: 4c 8b 7f 10 mov 0x10(%rdi),%r15 46139: 49 83 ff 06 cmp $0x6,%r15 4613d: 72 49 jb 46188 <bar_string+0x58> 4613f: 49 89 fe mov %rdi,%r14 46142: 48 8b 07 mov (%rdi),%rax 46145: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 46149: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 4614d: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 46151: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 46158: 00 00 00 4615b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 46160: 49 83 c7 ff add $0xffffffffffffffff,%r15 46164: 4d 89 7e 10 mov %r15,0x10(%r14) 46168: 48 8b 33 mov (%rbx),%rsi 4616b: 48 85 f6 test %rsi,%rsi 4616e: 74 0e je 4617e <bar_string+0x4e> 46170: 48 8b 7b f8 mov -0x8(%rbx),%rdi 46174: ba 01 00 00 00 mov $0x1,%edx 46179: e8 42 e9 ff ff callq 44ac0 <__rust_dealloc@plt> 4617e: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 46182: 49 83 ff 05 cmp $0x5,%r15 46186: 77 d8 ja 46160 <bar_string+0x30> 46188: 5b pop %rbx 46189: 41 5e pop %r14 4618b: 41 5f pop %r15 4618d: c3 retq 4618e: 66 90 xchg %ax,%ax 0000000000046190 <baz_string>: 46190: 41 57 push %r15 46192: 41 56 push %r14 46194: 41 54 push %r12 46196: 53 push %rbx 46197: 50 push %rax 46198: 4c 8b 67 10 mov 0x10(%rdi),%r12 4619c: 49 39 f4 cmp %rsi,%r12 4619f: 76 46 jbe 461e7 <baz_string+0x57> 461a1: 49 89 f6 mov %rsi,%r14 461a4: 49 89 ff mov %rdi,%r15 461a7: 48 8b 07 mov (%rdi),%rax 461aa: 4b 8d 0c 64 lea (%r12,%r12,2),%rcx 461ae: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 461b2: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 461b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 461bd: 00 00 00 461c0: 49 83 c4 ff add $0xffffffffffffffff,%r12 461c4: 4d 89 67 10 mov %r12,0x10(%r15) 461c8: 48 8b 33 mov (%rbx),%rsi 461cb: 48 85 f6 test %rsi,%rsi 461ce: 74 0e je 461de <baz_string+0x4e> 461d0: 48 8b 7b f8 mov -0x8(%rbx),%rdi 461d4: ba 01 00 00 00 mov $0x1,%edx 461d9: e8 e2 e8 ff ff callq 44ac0 <__rust_dealloc@plt> 461de: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 461e2: 4d 39 f4 cmp %r14,%r12 461e5: 77 d9 ja 461c0 <baz_string+0x30> 461e7: 48 83 c4 08 add $0x8,%rsp 461eb: 5b pop %rbx 461ec: 41 5c pop %r12 461ee: 41 5e pop %r14 461f0: 41 5f pop %r15 461f2: c3 retq 461f3: 90 nop 461f4: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 461fb: 00 00 00 461fe: 66 90 xchg %ax,%ax ``` </details> <details> <summary>New output</summary> ```asm 0000000000084d10 <foo>: 84d10: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi) 84d17: 00 84d18: c3 retq 84d19: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 0000000000084d20 <bar>: 84d20: 48 8b 47 10 mov 0x10(%rdi),%rax 84d24: 48 83 f8 05 cmp $0x5,%rax 84d28: b9 05 00 00 00 mov $0x5,%ecx 84d2d: 48 0f 42 c8 cmovb %rax,%rcx 84d31: 48 89 4f 10 mov %rcx,0x10(%rdi) 84d35: c3 retq 84d36: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84d3d: 00 00 00 0000000000084d40 <baz>: 84d40: 48 8b 47 10 mov 0x10(%rdi),%rax 84d44: 48 39 f0 cmp %rsi,%rax 84d47: 48 0f 47 c6 cmova %rsi,%rax 84d4b: 48 89 47 10 mov %rax,0x10(%rdi) 84d4f: c3 retq 0000000000084d50 <foo_string>: 84d50: 41 57 push %r15 84d52: 41 56 push %r14 84d54: 53 push %rbx 84d55: 49 89 fe mov %rdi,%r14 84d58: 4c 8b 7f 10 mov 0x10(%rdi),%r15 84d5c: 4d 85 ff test %r15,%r15 84d5f: 74 2f je 84d90 <foo_string+0x40> 84d61: 49 8b 06 mov (%r14),%rax 84d64: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 84d68: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 84d6c: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84d70: 48 8b 33 mov (%rbx),%rsi 84d73: 48 85 f6 test %rsi,%rsi 84d76: 74 0e je 84d86 <foo_string+0x36> 84d78: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84d7c: ba 01 00 00 00 mov $0x1,%edx 84d81: e8 1a b1 ff ff callq 7fea0 <__rust_dealloc@plt> 84d86: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84d8a: 49 83 c7 ff add $0xffffffffffffffff,%r15 84d8e: 75 e0 jne 84d70 <foo_string+0x20> 84d90: 49 c7 46 10 00 00 00 movq $0x0,0x10(%r14) 84d97: 00 84d98: 5b pop %rbx 84d99: 41 5e pop %r14 84d9b: 41 5f pop %r15 84d9d: c3 retq 84d9e: 66 90 xchg %ax,%ax 0000000000084da0 <bar_string>: 84da0: 41 57 push %r15 84da2: 41 56 push %r14 84da4: 53 push %rbx 84da5: 49 89 fe mov %rdi,%r14 84da8: 4c 8b 7f 10 mov 0x10(%rdi),%r15 84dac: 49 83 ff 06 cmp $0x6,%r15 84db0: 72 44 jb 84df6 <bar_string+0x56> 84db2: 49 8b 06 mov (%r14),%rax 84db5: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 84db9: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 84dbd: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84dc1: 49 83 c7 fb add $0xfffffffffffffffb,%r15 84dc5: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84dcc: 00 00 00 84dcf: 90 nop 84dd0: 48 8b 33 mov (%rbx),%rsi 84dd3: 48 85 f6 test %rsi,%rsi 84dd6: 74 0e je 84de6 <bar_string+0x46> 84dd8: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84ddc: ba 01 00 00 00 mov $0x1,%edx 84de1: e8 ba b0 ff ff callq 7fea0 <__rust_dealloc@plt> 84de6: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84dea: 49 83 c7 ff add $0xffffffffffffffff,%r15 84dee: 75 e0 jne 84dd0 <bar_string+0x30> 84df0: 41 bf 05 00 00 00 mov $0x5,%r15d 84df6: 4d 89 7e 10 mov %r15,0x10(%r14) 84dfa: 5b pop %rbx 84dfb: 41 5e pop %r14 84dfd: 41 5f pop %r15 84dff: c3 retq 0000000000084e00 <baz_string>: 84e00: 41 57 push %r15 84e02: 41 56 push %r14 84e04: 41 54 push %r12 84e06: 53 push %rbx 84e07: 50 push %rax 84e08: 49 89 ff mov %rdi,%r15 84e0b: 48 8b 47 10 mov 0x10(%rdi),%rax 84e0f: 49 89 c4 mov %rax,%r12 84e12: 49 29 f4 sub %rsi,%r12 84e15: 76 3c jbe 84e53 <baz_string+0x53> 84e17: 49 89 f6 mov %rsi,%r14 84e1a: 49 8b 0f mov (%r15),%rcx 84e1d: 48 8d 04 40 lea (%rax,%rax,2),%rax 84e21: 48 8d 1c c1 lea (%rcx,%rax,8),%rbx 84e25: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84e29: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 84e30: 48 8b 33 mov (%rbx),%rsi 84e33: 48 85 f6 test %rsi,%rsi 84e36: 74 0e je 84e46 <baz_string+0x46> 84e38: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84e3c: ba 01 00 00 00 mov $0x1,%edx 84e41: e8 5a b0 ff ff callq 7fea0 <__rust_dealloc@plt> 84e46: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84e4a: 49 83 c4 ff add $0xffffffffffffffff,%r12 84e4e: 75 e0 jne 84e30 <baz_string+0x30> 84e50: 4c 89 f0 mov %r14,%rax 84e53: 49 89 47 10 mov %rax,0x10(%r15) 84e57: 48 83 c4 08 add $0x8,%rsp 84e5b: 5b pop %rbx 84e5c: 41 5c pop %r12 84e5e: 41 5e pop %r14 84e60: 41 5f pop %r15 84e62: c3 retq 84e63: 90 nop 84e64: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84e6b: 00 00 00 84e6e: 66 90 xchg %ax,%ax ``` </details> For calling `truncate` with non-zero lengths on non-`Drop` types, it seems that a redundant load and comparison gets replaced with an awkward sequence with a conditional move. In the unknown length case, the new code is no longer awkward. Maybe someone moderately proficient at assembly could tell if this looks like a win or not. --- This came up when discussing replacing `unsafe { vec.set_len(0) }` with `vec.clear()` in a project where the author was worried about potential performance degradation. It might be worth replacing some unsafe code, even it it's trivial to see that it's actually safe.
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…chton Use SetLenOnDrop in Vec::truncate() This avoids a redundant length check in some cases when calling `Vec::truncate` or `Vec::clear`. Fixes rust-lang#51802 Note that the generated code still seems suboptimal. I tested with the following functions: ```rust #[no_mangle] pub extern fn foo(x: &mut Vec<u8>) { x.clear(); } #[no_mangle] pub extern fn bar(x: &mut Vec<u8>) { x.truncate(5); } #[no_mangle] pub extern fn baz(x: &mut Vec<u8>, n: usize) { x.truncate(n); } #[no_mangle] pub extern fn foo_string(x: &mut Vec<String>) { x.clear(); } #[no_mangle] pub extern fn bar_string(x: &mut Vec<String>) { x.truncate(5); } #[no_mangle] pub extern fn baz_string(x: &mut Vec<String>, n: usize) { x.truncate(n); } ``` <details> <summary>Old output</summary> ```asm 00000000000460a0 <foo>: 460a0: 48 83 7f 10 00 cmpq $0x0,0x10(%rdi) 460a5: 74 08 je 460af <foo+0xf> 460a7: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi) 460ae: 00 460af: c3 retq 00000000000460b0 <bar>: 460b0: 48 83 7f 10 06 cmpq $0x6,0x10(%rdi) 460b5: 72 08 jb 460bf <bar+0xf> 460b7: 48 c7 47 10 05 00 00 movq $0x5,0x10(%rdi) 460be: 00 460bf: c3 retq 00000000000460c0 <baz>: 460c0: 48 39 77 10 cmp %rsi,0x10(%rdi) 460c4: 76 04 jbe 460ca <baz+0xa> 460c6: 48 89 77 10 mov %rsi,0x10(%rdi) 460ca: c3 retq 460cb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 00000000000460d0 <foo_string>: 460d0: 41 57 push %r15 460d2: 41 56 push %r14 460d4: 53 push %rbx 460d5: 48 8b 47 10 mov 0x10(%rdi),%rax 460d9: 48 85 c0 test %rax,%rax 460dc: 74 4a je 46128 <foo_string+0x58> 460de: 49 89 fe mov %rdi,%r14 460e1: 48 8b 0f mov (%rdi),%rcx 460e4: 48 8d 14 40 lea (%rax,%rax,2),%rdx 460e8: 48 8d 58 ff lea -0x1(%rax),%rbx 460ec: 4c 8d 3c d1 lea (%rcx,%rdx,8),%r15 460f0: 49 83 c7 f0 add $0xfffffffffffffff0,%r15 460f4: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 460fb: 00 00 00 460fe: 66 90 xchg %ax,%ax 46100: 49 89 5e 10 mov %rbx,0x10(%r14) 46104: 49 8b 37 mov (%r15),%rsi 46107: 48 85 f6 test %rsi,%rsi 4610a: 74 0e je 4611a <foo_string+0x4a> 4610c: 49 8b 7f f8 mov -0x8(%r15),%rdi 46110: ba 01 00 00 00 mov $0x1,%edx 46115: e8 a6 e9 ff ff callq 44ac0 <__rust_dealloc@plt> 4611a: 48 83 c3 ff add $0xffffffffffffffff,%rbx 4611e: 49 83 c7 e8 add $0xffffffffffffffe8,%r15 46122: 48 83 fb ff cmp $0xffffffffffffffff,%rbx 46126: 75 d8 jne 46100 <foo_string+0x30> 46128: 5b pop %rbx 46129: 41 5e pop %r14 4612b: 41 5f pop %r15 4612d: c3 retq 4612e: 66 90 xchg %ax,%ax 0000000000046130 <bar_string>: 46130: 41 57 push %r15 46132: 41 56 push %r14 46134: 53 push %rbx 46135: 4c 8b 7f 10 mov 0x10(%rdi),%r15 46139: 49 83 ff 06 cmp $0x6,%r15 4613d: 72 49 jb 46188 <bar_string+0x58> 4613f: 49 89 fe mov %rdi,%r14 46142: 48 8b 07 mov (%rdi),%rax 46145: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 46149: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 4614d: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 46151: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 46158: 00 00 00 4615b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 46160: 49 83 c7 ff add $0xffffffffffffffff,%r15 46164: 4d 89 7e 10 mov %r15,0x10(%r14) 46168: 48 8b 33 mov (%rbx),%rsi 4616b: 48 85 f6 test %rsi,%rsi 4616e: 74 0e je 4617e <bar_string+0x4e> 46170: 48 8b 7b f8 mov -0x8(%rbx),%rdi 46174: ba 01 00 00 00 mov $0x1,%edx 46179: e8 42 e9 ff ff callq 44ac0 <__rust_dealloc@plt> 4617e: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 46182: 49 83 ff 05 cmp $0x5,%r15 46186: 77 d8 ja 46160 <bar_string+0x30> 46188: 5b pop %rbx 46189: 41 5e pop %r14 4618b: 41 5f pop %r15 4618d: c3 retq 4618e: 66 90 xchg %ax,%ax 0000000000046190 <baz_string>: 46190: 41 57 push %r15 46192: 41 56 push %r14 46194: 41 54 push %r12 46196: 53 push %rbx 46197: 50 push %rax 46198: 4c 8b 67 10 mov 0x10(%rdi),%r12 4619c: 49 39 f4 cmp %rsi,%r12 4619f: 76 46 jbe 461e7 <baz_string+0x57> 461a1: 49 89 f6 mov %rsi,%r14 461a4: 49 89 ff mov %rdi,%r15 461a7: 48 8b 07 mov (%rdi),%rax 461aa: 4b 8d 0c 64 lea (%r12,%r12,2),%rcx 461ae: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 461b2: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 461b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 461bd: 00 00 00 461c0: 49 83 c4 ff add $0xffffffffffffffff,%r12 461c4: 4d 89 67 10 mov %r12,0x10(%r15) 461c8: 48 8b 33 mov (%rbx),%rsi 461cb: 48 85 f6 test %rsi,%rsi 461ce: 74 0e je 461de <baz_string+0x4e> 461d0: 48 8b 7b f8 mov -0x8(%rbx),%rdi 461d4: ba 01 00 00 00 mov $0x1,%edx 461d9: e8 e2 e8 ff ff callq 44ac0 <__rust_dealloc@plt> 461de: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 461e2: 4d 39 f4 cmp %r14,%r12 461e5: 77 d9 ja 461c0 <baz_string+0x30> 461e7: 48 83 c4 08 add $0x8,%rsp 461eb: 5b pop %rbx 461ec: 41 5c pop %r12 461ee: 41 5e pop %r14 461f0: 41 5f pop %r15 461f2: c3 retq 461f3: 90 nop 461f4: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 461fb: 00 00 00 461fe: 66 90 xchg %ax,%ax ``` </details> <details> <summary>New output</summary> ```asm 0000000000084d10 <foo>: 84d10: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi) 84d17: 00 84d18: c3 retq 84d19: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 0000000000084d20 <bar>: 84d20: 48 8b 47 10 mov 0x10(%rdi),%rax 84d24: 48 83 f8 05 cmp $0x5,%rax 84d28: b9 05 00 00 00 mov $0x5,%ecx 84d2d: 48 0f 42 c8 cmovb %rax,%rcx 84d31: 48 89 4f 10 mov %rcx,0x10(%rdi) 84d35: c3 retq 84d36: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84d3d: 00 00 00 0000000000084d40 <baz>: 84d40: 48 8b 47 10 mov 0x10(%rdi),%rax 84d44: 48 39 f0 cmp %rsi,%rax 84d47: 48 0f 47 c6 cmova %rsi,%rax 84d4b: 48 89 47 10 mov %rax,0x10(%rdi) 84d4f: c3 retq 0000000000084d50 <foo_string>: 84d50: 41 57 push %r15 84d52: 41 56 push %r14 84d54: 53 push %rbx 84d55: 49 89 fe mov %rdi,%r14 84d58: 4c 8b 7f 10 mov 0x10(%rdi),%r15 84d5c: 4d 85 ff test %r15,%r15 84d5f: 74 2f je 84d90 <foo_string+0x40> 84d61: 49 8b 06 mov (%r14),%rax 84d64: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 84d68: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 84d6c: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84d70: 48 8b 33 mov (%rbx),%rsi 84d73: 48 85 f6 test %rsi,%rsi 84d76: 74 0e je 84d86 <foo_string+0x36> 84d78: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84d7c: ba 01 00 00 00 mov $0x1,%edx 84d81: e8 1a b1 ff ff callq 7fea0 <__rust_dealloc@plt> 84d86: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84d8a: 49 83 c7 ff add $0xffffffffffffffff,%r15 84d8e: 75 e0 jne 84d70 <foo_string+0x20> 84d90: 49 c7 46 10 00 00 00 movq $0x0,0x10(%r14) 84d97: 00 84d98: 5b pop %rbx 84d99: 41 5e pop %r14 84d9b: 41 5f pop %r15 84d9d: c3 retq 84d9e: 66 90 xchg %ax,%ax 0000000000084da0 <bar_string>: 84da0: 41 57 push %r15 84da2: 41 56 push %r14 84da4: 53 push %rbx 84da5: 49 89 fe mov %rdi,%r14 84da8: 4c 8b 7f 10 mov 0x10(%rdi),%r15 84dac: 49 83 ff 06 cmp $0x6,%r15 84db0: 72 44 jb 84df6 <bar_string+0x56> 84db2: 49 8b 06 mov (%r14),%rax 84db5: 4b 8d 0c 7f lea (%r15,%r15,2),%rcx 84db9: 48 8d 1c c8 lea (%rax,%rcx,8),%rbx 84dbd: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84dc1: 49 83 c7 fb add $0xfffffffffffffffb,%r15 84dc5: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84dcc: 00 00 00 84dcf: 90 nop 84dd0: 48 8b 33 mov (%rbx),%rsi 84dd3: 48 85 f6 test %rsi,%rsi 84dd6: 74 0e je 84de6 <bar_string+0x46> 84dd8: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84ddc: ba 01 00 00 00 mov $0x1,%edx 84de1: e8 ba b0 ff ff callq 7fea0 <__rust_dealloc@plt> 84de6: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84dea: 49 83 c7 ff add $0xffffffffffffffff,%r15 84dee: 75 e0 jne 84dd0 <bar_string+0x30> 84df0: 41 bf 05 00 00 00 mov $0x5,%r15d 84df6: 4d 89 7e 10 mov %r15,0x10(%r14) 84dfa: 5b pop %rbx 84dfb: 41 5e pop %r14 84dfd: 41 5f pop %r15 84dff: c3 retq 0000000000084e00 <baz_string>: 84e00: 41 57 push %r15 84e02: 41 56 push %r14 84e04: 41 54 push %r12 84e06: 53 push %rbx 84e07: 50 push %rax 84e08: 49 89 ff mov %rdi,%r15 84e0b: 48 8b 47 10 mov 0x10(%rdi),%rax 84e0f: 49 89 c4 mov %rax,%r12 84e12: 49 29 f4 sub %rsi,%r12 84e15: 76 3c jbe 84e53 <baz_string+0x53> 84e17: 49 89 f6 mov %rsi,%r14 84e1a: 49 8b 0f mov (%r15),%rcx 84e1d: 48 8d 04 40 lea (%rax,%rax,2),%rax 84e21: 48 8d 1c c1 lea (%rcx,%rax,8),%rbx 84e25: 48 83 c3 f0 add $0xfffffffffffffff0,%rbx 84e29: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 84e30: 48 8b 33 mov (%rbx),%rsi 84e33: 48 85 f6 test %rsi,%rsi 84e36: 74 0e je 84e46 <baz_string+0x46> 84e38: 48 8b 7b f8 mov -0x8(%rbx),%rdi 84e3c: ba 01 00 00 00 mov $0x1,%edx 84e41: e8 5a b0 ff ff callq 7fea0 <__rust_dealloc@plt> 84e46: 48 83 c3 e8 add $0xffffffffffffffe8,%rbx 84e4a: 49 83 c4 ff add $0xffffffffffffffff,%r12 84e4e: 75 e0 jne 84e30 <baz_string+0x30> 84e50: 4c 89 f0 mov %r14,%rax 84e53: 49 89 47 10 mov %rax,0x10(%r15) 84e57: 48 83 c4 08 add $0x8,%rsp 84e5b: 5b pop %rbx 84e5c: 41 5c pop %r12 84e5e: 41 5e pop %r14 84e60: 41 5f pop %r15 84e62: c3 retq 84e63: 90 nop 84e64: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 84e6b: 00 00 00 84e6e: 66 90 xchg %ax,%ax ``` </details> For calling `truncate` with non-zero lengths on non-`Drop` types, it seems that a redundant load and comparison gets replaced with an awkward sequence with a conditional move. In the unknown length case, the new code is no longer awkward. Maybe someone moderately proficient at assembly could tell if this looks like a win or not. --- This came up when discussing replacing `unsafe { vec.set_len(0) }` with `vec.clear()` in a project where the author was worried about potential performance degradation. It might be worth replacing some unsafe code, even it it's trivial to see that it's actually safe.
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See https://play.rust-lang.org/?gist=2754f11f7236754b2121eb9e4c12aa73&version=nightly&mode=release. The length of the
Vecis read before being written to.I would have expected the peephole optimizer in LLVM to recognize that pattern. Interestingly, it's not a regression; according to https://rust.godbolt.org/, the issue was even present in
rustc 1.0.The text was updated successfully, but these errors were encountered: