codegen: store ScalarPair via memset when one side is undef and the other side can be memset

[oli-obk]

Basically since undef can be any byte, it can also be the byte(s) that are in the non-undef parts of a value. So we can just treat the undef at not being there and only look at the initialized bytes and memset over them

fixes #104290

based on #135258

[rustbot]

r? @wesleywiser

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[rustbot]

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @rust-lang/wg-const-eval

[oli-obk]

@bors try @rust-timer queue

cc @saethlin in case you were interested in where #135258 was going

[bors]

⌛ Trying commit e64a5a0 with merge ca1798b...

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[lqd]

@bors try

[bors]

⌛ Trying commit b7e20ee with merge cabcef2...

[bors]

☀️ Try build successful - checks-actions
Build commit: cabcef2 (cabcef235464ac1ed5f06ae4092950ae5027f6e2)

[rust-timer]

Finished benchmarking commit (cabcef2): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -2.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary -0.1%, secondary -0.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	53
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	14
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	53

Bootstrap: 763.776s -> 764.773s (0.13%)
Artifact size: 326.00 MiB -> 325.98 MiB (-0.01%)

[saethlin]

Widespread (even though small) binary size improvements in debug builds indicate this is firing and decently relevant.

[RalfJung]

I have updated the PR title since I found the old one really confusing.^^

[oli-obk]

Well my original plan was to do this for general Ref constants, too, and I have that commit locally

[RalfJung]

Feel free to further adjust the title. The old title was extremely unspecific though.^^

[saethlin]

r? saethlin
@bors r+
🚀

[bors]

📌 Commit 83476be has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 83476be with merge 663dd5e...

[bors]

💔 Test failed - checks-actions

[oli-obk]

@bors r=saethlin

[bors]

📌 Commit 8f5f5e5 has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8f5f5e5 with merge a7a6c64...

[bors]

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing a7a6c64 to master...

[rust-timer]

Finished benchmarking commit (a7a6c64): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary -2.2%, secondary 1.6%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.2%	[1.2%, 1.2%]	1
Regressions ❌ (secondary)	1.6%	[1.5%, 1.6%]	3
Improvements ✅ (primary)	-4.0%	[-5.2%, -2.8%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.2%	[-5.2%, 1.2%]	3

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary -0.1%, secondary -0.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	53
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	14
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	53

Bootstrap: 763.258s -> 763.46s (0.03%)
Artifact size: 326.07 MiB -> 326.05 MiB (-0.01%)