[x86 PATCH] Tweak ix86_expand_int_compare to use PTEST for vector equality.

[Roger Sayle]

 

Hi Uros,

 

Hopefully Hongtao will approve my patch to support SUBREG conversions

in STV https://gcc.gnu.org/pipermail/gcc-patches/2023-June/622706.html

but for some of the examples described in the above post (and its test

case), I've also come up with an alternate/complementary/supplementary

fix of generating the PTEST during RTL expansion, rather than rely on

this being caught/optimized later during STV.

 

You may notice in this patch, the tests for TARGET_SSE4_1 and TImode

appear last.  When I was writing this, I initially also added support

for AVX VPTEST and OImode, before realizing that x86 doesn't (yet)

support 256-bit OImode (which also explains why we don't have an OImode

to V1OImode scalar-to-vector pass).  Retaining this clause ordering

should minimize the lines changed if things change in future.

 

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap

and make -k check, both with and without --target_board=unix{-m32}

with no new failures.  Ok for mainline?

 

 

2023-06-27  Roger Sayle  <ro...@nextmovesoftware.com>

 

gcc/ChangeLog

        * config/i386/i386-expand.cc (ix86_expand_int_compare): If

        testing a TImode SUBREG of a 128-bit vector register against

        zero, use a PTEST instruction instead of first moving it to

        to scalar registers.

 

 

Please let me know what you think.

Roger

--

 


diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-features.cc
index 4a3b07a..53bec08 100644
--- a/gcc/config/i386/i386-features.cc
+++ b/gcc/config/i386/i386-features.cc
@@ -631,7 +631,31 @@ general_scalar_chain::compute_convert_gain ()
            break;
 
          case COMPARE:
-           /* Assume comparison cost is the same.  */
+           if (XEXP (src, 1) != const0_rtx)
+             {
+               /* cmp vs. pxor;pshufd;ptest.  */
+               igain += COSTS_N_INSNS (m - 3);
+             }
+           else if (GET_CODE (XEXP (src, 0)) != AND)
+             {
+               /* test vs. pshufd;ptest.  */
+               igain += COSTS_N_INSNS (m - 2);
+             }
+           else if (GET_CODE (XEXP (XEXP (src, 0), 0)) != NOT)
+             {
+               /* and;test vs. pshufd;ptest.  */
+               igain += COSTS_N_INSNS (2 * m - 2);
+             }
+           else if (TARGET_BMI)
+             {
+               /* andn;test vs. pandn;pshufd;ptest.  */
+               igain += COSTS_N_INSNS (2 * m - 3);
+             }
+           else
+             {
+               /* not;and;test vs. pandn;pshufd;ptest.  */
+               igain += COSTS_N_INSNS (3 * m - 3);
+             }
            break;
 
          case CONST_INT: