RE: [PATCH] gdb/record: avoid rax usage in i386-avx-record.exp

> -----Original Message-----
> From: Joos, Christina
> Sent: Mittwoch, 12. August 2026 15:31
> To: 'Guinevere Larsen' <[email protected]>; gdb-
> [email protected]
> Subject: RE: [PATCH] gdb/record: avoid rax usage in i386-avx-record.exp
> 
> > -----Original Message-----
> > From: Guinevere Larsen <[email protected]>
> > Sent: Montag, 10. August 2026 21:33
> > To: [email protected]
> > Cc: Guinevere Larsen <[email protected]>; Joos, Christina
> > <[email protected]>
> > Subject: [PATCH] gdb/record: avoid rax usage in i386-avx-record.exp
> >
> > My recent commit, 1c04308f0958cbac83bad22703758139ea6969b6:
> >
> >     Author: Guinevere Larsen <[email protected]>
> >     Date:   Tue Jun 16 16:03:43 2026 -0300
> >     gdb/record: add support for BMI2 instructions
> >
> > Introduced a few tests that rely on the rax register being set before
> > entering the function. This isn't reliable, as eax is almost always
> > set to 0 (except in my machine for some reason). To avoid ISAs getting
> > in the way, this commit changes the test to use r12 (and r12w/r12d
> > when appropriate), so that it won't be disturbed by the compiler.
> >
> > Reported-by: Christina Joos <[email protected]>
> > ---
> >  gdb/testsuite/gdb.reverse/i386-avx-reverse.c  | 32 +++++++++----------
> >  .../gdb.reverse/i386-avx-reverse.exp          |  8 ++---
> >  2 files changed, 20 insertions(+), 20 deletions(-)
> >
> > diff --git a/gdb/testsuite/gdb.reverse/i386-avx-reverse.c
> > b/gdb/testsuite/gdb.reverse/i386-avx-reverse.c
> > index 88cd3fc7f34..4d45482c07a 100644
> > --- a/gdb/testsuite/gdb.reverse/i386-avx-reverse.c
> > +++ b/gdb/testsuite/gdb.reverse/i386-avx-reverse.c
> > @@ -770,48 +770,48 @@ bmi_test ()
> >  {
> >    /* start bmi_test.  */
> >    /* Using GDB, load these values onto registers for testing.
> > -     eax = 0x1312
> > +     r12 = 0x1312
> >       ebx = 0xdeadbeef
> >       ecx = 0xcafeface
> >       edx = 0x2337
> >       r8  = 0
> >       this way it's easy to confirm we're undoing things correctly.
> > */
> >
> > -  asm volatile ("andn %rbx, %rax, %r8");
> > -  asm volatile ("andn %ebx, %eax, %ecx");
> > +  asm volatile ("andn %rbx, %r12, %r8");  asm volatile ("andn %ebx,
> > + %r12d, %ecx");
> >
> >    asm volatile ("bextr %rcx, %r8, %rbx");
> >    asm volatile ("bextr %ebx, %ecx, %r8d");
> >
> > -  asm volatile ("blsi %rax, %rcx");
> > +  asm volatile ("blsi %r12, %rcx");
> >    asm volatile ("blsi %ebx, %r8d");
> >
> >    asm volatile ("blsmsk %r8, %rbx");
> > -  asm volatile ("blsmsk %eax, %eax");
> > +  asm volatile ("blsmsk %r12d, %r12d");
> >
> >    asm volatile ("blsr %rcx, %rbx");
> >    asm volatile ("blsr %r8d, %ecx");
> >
> > -  asm volatile ("lzcnt %rax, %r8");
> > -  asm volatile ("lzcnt %eax, %ecx");
> > -  asm volatile ("lzcnt %ax, %bx");
> > +  asm volatile ("lzcnt %r12, %r8");
> > +  asm volatile ("lzcnt %r12d, %ecx");  asm volatile ("lzcnt %r12w,
> > + %bx");
> >
> > -  asm volatile ("tzcnt %rax, %rcx");
> > -  asm volatile ("tzcnt %eax, %ebx");
> > -  asm volatile ("tzcnt %ax, %r8w");
> > +  asm volatile ("tzcnt %r12, %rcx");
> > +  asm volatile ("tzcnt %r12d, %ebx");  asm volatile ("tzcnt %r12w,
> > + %r8w");
> >
> > -  asm volatile ("mulx %eax, %ebx, %ecx");
> > +  asm volatile ("mulx %r12d, %ebx, %ecx");
> >    asm volatile ("mulx %ebx, %ecx, %ebx");
> >
> >    asm volatile ("pdep %r8, %rcx, %rbx");
> >    asm volatile ("pext %edx, %ecx, %ebx");
> >    asm volatile ("rorx $10, %r8, %rbx");
> > -  asm volatile ("sarx %edx, %ebx, %eax");
> > -  asm volatile ("shlx %eax, %ecx, %ebx");
> > -  asm volatile ("shrx %r8d, %eax, %ecx");
> > +  asm volatile ("sarx %edx, %ebx, %r12d");  asm volatile ("shlx
> > + %r12d, %ecx, %ebx");  asm volatile ("shrx %r8d, %r12d, %ecx");
> >
> >    asm volatile ("bzhi %edx, %ebx, %r8d");
> > -  asm volatile ("bzhi %rax, %rcx, %r8");
> > +  asm volatile ("bzhi %r12, %rcx, %r8");
> >
> >    return 0; /* end bmi_test  */
> >  }
> > diff --git a/gdb/testsuite/gdb.reverse/i386-avx-reverse.exp
> > b/gdb/testsuite/gdb.reverse/i386-avx-reverse.exp
> > index 5d65245db39..a5b0e6612aa 100644
> > --- a/gdb/testsuite/gdb.reverse/i386-avx-reverse.exp
> > +++ b/gdb/testsuite/gdb.reverse/i386-avx-reverse.exp
> > @@ -1201,7 +1201,7 @@ gdb_test "finish" "Run till exit
> > from.*convert_test.*" \ gdb_test_no_output \
> >      "set \$r8 = 0" "set r8 for bmi test"
> >  gdb_test_no_output \
> > -    "set \$eax = 0x1312" "set eax for bmi test"
> > +    "set \$r12 = 0x1312" "set r12 for bmi test"
> >  gdb_test_no_output "set \$ebx = 0xdeadbeef" "set ebx for bmi test"
> >  gdb_test_no_output "set \$ecx = 0xcafeface" "set ecx for bmi test"
> >  gdb_test_no_output "set \$edx = 0x2337" "set edx for bmi test"
> > @@ -1213,7 +1213,7 @@ if {[record_full_function "bmi"] == true} {
> >
> >      test_one_general_register "shrx" "ecx" "0xe884173"
> >      test_one_general_register "shlx" "ebx" "0x0"
> > -    test_one_general_register "sarx" "eax" "0x3"
> > +    test_one_general_register "sarx" "r12" "0x3"
> >      test_one_general_register "rorx" "ebx" "0x3b"
> >      test_one_general_register "pext" "ebx" "0x0"
> >      test_one_general_register "pdep" "ebx" "0x0"
> > @@ -1232,13 +1232,13 @@ if {[record_full_function "bmi"] == true} {
> >      test_one_general_register "blsr" "ecx" "0x2"
> >      test_one_general_register "blsr" "ebx" "0xffffffff"
> >
> > -    test_one_general_register "blsmsk" "eax" "0x1312"
> > +    test_one_general_register "blsmsk" "r12" "0x1312"
> >      test_one_general_register "blsmsk" "ebx" "0x0"
> >
> >      test_one_general_register "blsi" "r8" "0x0"
> >      test_one_general_register "blsi" "ecx" "0xdeadaced"
> >
> > -    test_one_general_register "bextr" "eax" "0x1312"
> > +    test_one_general_register "bextr" "r12" "0x1312"
> >      test_one_general_register "bextr" "ebx" "0xdeadbeef"
> >
> >      test_one_general_register "andn" "ecx" "0xcafeface"
> >
> > base-commit: a80fede20bc1330eca5e419392c6595bb3a6ac1d
> > --
> > 2.55.0
> 
> Hi Guinevere,
> 
> Thank you for this patch.
> My machine has been updated recently to Ubuntu 26.04 and now the test
> passes also for me (without your patch).
> Reason is that I now use a newer gcc version (gcc-15) by default.
> 
> Using gcc-14 it still fails, so maybe it would make sense to mention that in the
> commit message.
> 
> I still think using callee saved registers in the entire test would be better, also to
> avoid future mistakes.
> But since it's now passing again, I guess this is also fine.
> 
> So with that fixed:

This comment might not be so clear about "that", so to clarify:

With a comment about the compiler in the commit message. 

Reviewed-by: Christina Joos <[email protected]>

Kind Regards,
Christina
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