When dynamic linking, R15 may be clobbered by a global variable access

[gbotrel]

See also #112 .

Can reproduce on x86 machine with

go build -linkshared ./...

For small moduli (<= 5 words) we can avoid the use of R15 register, a fix will be pushed in the code generator.

For larger moduli:

• define qInv0 and qElement as constants through a define macro (imm)
• put qElement on the stack at the start of the function
• .. ?

however, for large moduli, this impact performance. Tracking the issue here, one option would be to use build tags for dynamic linking and a slower path.

[ale-linux]

The issue seems to be back

$ go build -linkshared ./...
# github.com/consensys/gnark-crypto/ecc/bn254/fr/mimc/test_vectors
/home/aso/local/go/pkg/tool/linux_amd64/link: cannot implicitly include runtime/cgo in a shared library
# github.com/consensys/gnark-crypto/ecc/bls12-377/fp
asm: element_mul_amd64.s:93: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00012 (/home/aso/w/Projects/OBC/Workspace/src/github.com/Consensys/gnark-crypto/ecc/bls12-377/fp/element_mul_amd64.s:93)      MULXQ   R10, R14, R15
asm: assembly failed
# github.com/consensys/gnark-crypto/ecc/bls12-378/fp
asm: element_mul_amd64.s:93: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00012 (/home/aso/w/Projects/OBC/Workspace/src/github.com/Consensys/gnark-crypto/ecc/bls12-378/fp/element_mul_amd64.s:93)      MULXQ   R10, R14, R15
asm: assembly failed
# github.com/consensys/gnark-crypto/ecc/bls12-381/fp
asm: element_mul_amd64.s:93: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00012 (/home/aso/w/Projects/OBC/Workspace/src/github.com/Consensys/gnark-crypto/ecc/bls12-381/fp/element_mul_amd64.s:93)      MULXQ   R10, R14, R15
asm: assembly failed
# github.com/consensys/gnark-crypto/ecc/bw6-761/fr
asm: element_mul_amd64.s:93: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00012 (/home/aso/w/Projects/OBC/Workspace/src/github.com/Consensys/gnark-crypto/ecc/bw6-761/fr/element_mul_amd64.s:93)        MULXQ   R10, R14, R15
asm: assembly failed
# github.com/consensys/gnark-crypto/ecc/bw6-756/fr
asm: element_mul_amd64.s:93: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00012 (/home/aso/w/Projects/OBC/Workspace/src/github.com/Consensys/gnark-crypto/ecc/bw6-756/fr/element_mul_amd64.s:93)        MULXQ   R10, R14, R15
asm: assembly failed
# github.com/consensys/gnark-crypto/ecc/bw6-633/fp
asm: element_mul_amd64.s:106: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00009 (/home/aso/w/Projects/OBC/Workspace/src/github.com/Consensys/gnark-crypto/ecc/bw6-633/fp/element_mul_amd64.s:106)      MULXQ   (R12), R14, R15
asm: assembly failed

[ale-linux]

@gbotrel ^