[Bug target/95566] New: x86 instruction selection --- some REX prefixes unnecessary

[Bug target/95566] New: x86 instruction selection --- some REX prefixes unnecessary

[zero at smallinteger dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95566

            Bug ID: 95566
           Summary: x86 instruction selection --- some REX prefixes
                    unnecessary
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zero at smallinteger dot com
  Target Milestone: ---

Created attachment 48696
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48696&action=edit
sample code

Consider the code attached, compiled with

gcc -O3 sample.c -o sample

Gcc produces unrolled loop code that follows the pattern below.

        movzx   ecx, WORD PTR [rsp-62]
        cmp     rdx, rcx

Here, rdx has the value of k >> 48.  The top 32 bits of rdx are zero after the
shift, so the entirety of k >> 48 is in edx.  Thus, the cmp instructions could
be

        cmp     edx, ecx

instead.  This difference avoids the REX prefix, and thus the instructions are
shorter.  After sufficient unrolling (or with e.g. more complex comparisons
that depend on k >> 48), shorter instructions without the REX prefix will be
better even accounting for the partial register dependency (or an instruction
to break the dependency).  The Intel optimization manual says shorter
instructions are better.

The attachment is the entirety of sample.c.  I did not include other files
because this attachment appears to qualify for that exemption due to excuse
(ii): the attached test case is small and does not include any other file.

I originally found this behavior looking at the disassembly of gcc (Gentoo
9.2.0-r2 p3) 9.2.0.  I verified the same behavior with gcc 10.1 and gcc trunk
at godbolt.

[Bug target/95566] x86 instruction selection --- some REX prefixes unnecessary

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95566

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-08-20
           Severity|normal                      |enhancement
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
int f(unsigned short *a, unsigned long long d)
{
    return *a == (d>>48);
}

---- CUT ----
of the compilers I have compared, only ICX can do this:
        shrq    $48, %rsi
        xorl    %eax, %eax
        cmpw    %si, (%rdi)
        sete    %al
        retq

[Bug target/95566] x86 instruction selection --- some REX prefixes unnecessary

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95566

Hongtao.liu <crazylht at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |crazylht at gmail dot com

--- Comment #2 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> Reduced testcase:
> int f(unsigned short *a, unsigned long long d)
> {
>     return *a == (d>>48);
> }
> 
> ---- CUT ----
> of the compilers I have compared, only ICX can do this:
>         shrq    $48, %rsi
>         xorl    %eax, %eax
>         cmpw    %si, (%rdi)
>         sete    %al
>         retq

Failed to match this instruction:
(set (reg:QI 93)
    (eq:QI (lshiftrt:DI (reg:DI 95)
            (const_int 48 [0x30]))
        (zero_extend:DI (mem:HI (reg:DI 94) [1 *a_6(D)+0 S2 A16]))))

guess we can drop the zero_extend here, but still 3 instruction vs 3
instruction, just some codesize optimization.