commit 060bf14d30f8a6b2e19c8aab764c104725b1682f
author Keith Randall <khr@golang.org> Thu Jun 22 15:18:25 2023 -0700
committer Keith Randall <khr@google.com> Mon Jul 03 19:24:36 2023 +0000
tree 61ed10c6c0163c64bbd4134b07a06c6cc24dcc4b
parent 40c19ba4a7c5ffc92baaef5977ffc2b82dfb0e47

x86asm: disassemble CMP instruction's arguments in the opposite order

That way it matches what the compiler's -S flag generates, and what we write
in assembly.

  CMP AX, $16
  JLE foo

should get to foo if AX <= 16. Without this CL, the disassembly looks like

  CMP $16, AX
  JLE foo

which reads like we should get to foo if 16 <= AX, which is not what these
two instructions actually do.

It was originally this way because the CMP instruction parallels the SUB
instruction, except it throws away the non-flags result. We write that
subtraction as

  SUB $16, AX  // AX <- AX-16

but we don't need to match the SUB's disassembly order, as CMP is not
writing to a register output.

Update golang/go#60920
(This fixes the underlying issue, but the actual "fixes" comment needs to go
on the CL that vendors x/arch containing this CL into the main branch.)

Change-Id: Ifa8d3878453d6e33ae144bfdb01b34171c2106a1
Reviewed-on: https://go-review.googlesource.com/c/arch/+/505375
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>