[Bug target/113729] New: Missing APX NDD optimization

[Bug target/113729] New: Missing APX NDD optimization

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113729

            Bug ID: 113729
           Summary: Missing APX NDD optimization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
                CC: crazylht at gmail dot com, hongyuw at gcc dot gnu.org
  Target Milestone: ---
            Target: x86-64

APX spec has

---
Unlike the merge-upper behavior at a destination register of a typical x86
integer instruction when OSIZE
is 8b or 16b, the NDD register is always zero-uppered
---

But GCC 14 generates:

[hjl@gnu-tgl-3 pr113711]$ cat b.c
extern unsigned char b;

unsigned int
foo (void)
{
  return 200 + b;
}
[hjl@gnu-tgl-3 pr113711]$ make b.s
/export/build/gnu/tools-build/gcc-gitlab-debug/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/tools-build/gcc-gitlab-debug/build-x86_64-linux/gcc/ -mapxf
-O3 -S b.c
[hjl@gnu-tgl-3 pr113711]$ cat b.s
        .file   "b.c"
        .text
        .p2align 4
        .globl  foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        movzbl  b(%rip), %eax
        addl    $200, %eax
        ret
        .cfi_endproc
.LFE0:
        .size   foo, .-foo
        .ident  "GCC: (GNU) 14.0.1 20240202 (experimental)"
        .section        .note.G
[hjl@gnu-tgl-3 pr113711]$

addb    $200, b(%rip), %al

should be generated.

[Bug target/113729] Missing APX NDD optimization

[liuhongt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113729

Hongtao Liu <liuhongt at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |liuhongt at gcc dot gnu.org

--- Comment #1 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
At gimple level it's (int) b + 200, it's different from (int)(b +200) when
there's overflow, i.e. b = 200. 

For
extern unsigned char b;

unsigned char
foo (void)
{
  return 200 + b;
}

gcc already generate
 sub al, BYTE PTR b[rip], 56

[Bug target/113729] Missing APX NDD optimization

[liuhongt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113729

--- Comment #2 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
extern unsigned char b;

int
foo (void)
{
  return (unsigned char)(200 + b);
}

gcc -O2 -mapxf 

foo():
  subb $56, b(%rip), %al
  movzbl %al, %eax
  ret

And this can be optimzied to

foo():
  subb $56, b(%rip), %al
  ret

Note, if we want to optimize it in pass_combine, the pattern need to generate
explicit APX NDD instructions, since APX non-NDD will not clear upper bits.