This implementation utilizes the same registers found in the reference
implementation, aiming to produce a minimal semantic diff between the
Avo-generated output and the original hand-written assembly.

To verify the Avo implementation, the reference and Avo-generated
assembly files are fed to `go tool asm`, capturing the debug output into
corresponding temp files. The debug output contains supplementary
metadata (line numbers, instruction offsets, and source file references)
that must be removed in order to obtain a semantic diff of the two
files. This is accomplished via a small utility script written in awk.

The reference assembly file does not specify a frame size for a number
of the defined assembly functions. Avo automatically infers the frame
size when generating the TEXT directive, leading to a diff on those
lines.

Commands used to verify Avo output:

GOROOT=$(go env GOROOT)
ASM_PATH="src/crypto/internal/nistec/p256_asm_amd64.s"
REFERENCE="54fe0fd43fcf8609666c16ae6d15ed92873b1564"

go tool asm -o /dev/null -I "$GOROOT"/src/runtime -debug \
  <(git cat-file -p "$REFERENCE:$ASM_PATH") \
  > /tmp/reference.s

go tool asm -o /dev/null -I "$GOROOT"/src/runtime -debug \
  "$ASM_PATH" \
  > /tmp/avo.s

normalize(){
  awk '{
    $1=$2=$3="";
    print substr($0,4)
  }'
}

diff <(normalize < /tmp/reference.s) <(normalize < /tmp/avo.s)

1c1
< TEXT <unlinkable>.p256OrdLittleToBig(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256OrdLittleToBig(SB), NOSPLIT, $0-16
3c3
< TEXT <unlinkable>.p256OrdBigToLittle(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256OrdBigToLittle(SB), NOSPLIT, $0-16
5c5
< TEXT <unlinkable>.p256LittleToBig(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256LittleToBig(SB), NOSPLIT, $0-16
7c7
< TEXT <unlinkable>.p256BigToLittle(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256BigToLittle(SB), NOSPLIT, $0-16
23c23
< TEXT <unlinkable>.p256MovCond(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256MovCond(SB), NOSPLIT, $0-32
74c74
< TEXT <unlinkable>.p256NegCond(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256NegCond(SB), NOSPLIT, $0-16
99c99
< TEXT <unlinkable>.p256Sqr(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256Sqr(SB), NOSPLIT, $0-24
234c234
< TEXT <unlinkable>.p256Mul(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256Mul(SB), NOSPLIT, $0-24
401c401
< TEXT <unlinkable>.p256FromMont(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256FromMont(SB), NOSPLIT, $0-16
465c465
< TEXT <unlinkable>.p256Select(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256Select(SB), NOSPLIT, $0-24
513c513
< TEXT <unlinkable>.p256SelectAffine(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256SelectAffine(SB), NOSPLIT, $0-24
566c566
< TEXT <unlinkable>.p256OrdMul(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256OrdMul(SB), NOSPLIT, $0-24
806c806
< TEXT <unlinkable>.p256OrdSqr(SB), NOSPLIT, $0
---
> TEXT <unlinkable>.p256OrdSqr(SB), NOSPLIT, $0-24

Change-Id: I610b097c573b9d9018f0e26bc2afde5edb3f954b
Reviewed-on: https://go-review.googlesource.com/c/go/+/599875
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>