[Bug target/114567] New: rs6000: explicit _Float128 doesn't generate optimal code

[Bug target/114567] New: rs6000: explicit _Float128 doesn't generate optimal code

[linkw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114567

            Bug ID: 114567
           Summary: rs6000: explicit _Float128 doesn't generate optimal
                    code
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linkw at gcc dot gnu.org
  Target Milestone: ---

This is an issue which I happened to spot when I have been working on patches
for PR112993.

=== test case ===

#define TYPE _Float128

#ifdef LD
#undef TYPE
#define TYPE long double
#endif

int sbm (TYPE *a) { return __builtin_signbit (*a); }

==================

/opt/gcc-nightly/trunk/bin/gcc -mcpu=power8 -mvsx -O2 -mabi=ieeelongdouble
-Wno-psabi test.c -DLD -S -o ref.s
/opt/gcc-nightly/trunk/bin/gcc -mcpu=power8 -mvsx -O2 -mabi=ibmlongdouble
-Wno-psabi test.c -S -o float128.s

diff -Nur ref.s float128.s
--- ref.s       2024-03-18 05:41:00.302208975 -0400
+++ float128.s  2024-03-18 05:41:00.392205513 -0400
@@ -9,7 +9,10 @@
 sbm:
 .LFB0:
        .cfi_startproc
-       ld 3,8(3)
+       rldicr 3,3,0,59
+       lxvd2x 0,0,3
+       xxpermdi 0,0,0,2
+       mfvsrd 3,0
        srdi 3,3,63
        blr
        .long 0

[Bug target/114567] rs6000: explicit _Float128 doesn't generate optimal code

[linkw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114567

Kewen Lin <linkw at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |15.0
           Keywords|                            |missed-optimization
             Target|                            |powerpc64*-linux-gnu
   Last reconfirmed|                            |2024-04-03
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |linkw at gcc dot gnu.org

[Bug target/114567] rs6000: explicit _Float128 doesn't generate optimal code

[linkw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114567

--- Comment #1 from Kewen Lin <linkw at gcc dot gnu.org> ---
This is power8 LE specific, for KFmode its mov expander calls
rs6000_emit_le_vsx_move, so it's with V1TI subreg, then rs6000 specific pass
swaps generate one MEM with AND -16, which make combine unable to optimize it
with that *signbit<mode>2_dm_mem pattern due to mode_dependent_address_p
returning false always for AND. Although it looks to me we can extend
mode_dependent_address_p to consider the to-mode in that context, it's still
sub-optimal due to the existence of AND -16, which result in an explicit "and"
then.