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From: Aldy Hernandez <aldyh@gcc.gnu.org>
To: gcc-cvs@gcc.gnu.org
Subject: [gcc r13-2265] A == 0 ? A : -A    same as -A (when A is 0.0)
X-Act-Checkin: gcc
X-Git-Author: Aldy Hernandez <aldyh@redhat.com>
X-Git-Refname: refs/heads/master
X-Git-Oldrev: 34ad7155fe2a4a5d0b1815c79ce8b2af4c772c9e
X-Git-Newrev: df8fe4adb0721ab0e4486bc58482b501fe06287d
Message-Id: <20220830092742.C2F14385C335@sourceware.org>
Date: Tue, 30 Aug 2022 09:27:42 +0000 (GMT)
List-Id: <gcc-cvs.sourceware.org>

https://gcc.gnu.org/g:df8fe4adb0721ab0e4486bc58482b501fe06287d

commit r13-2265-gdf8fe4adb0721ab0e4486bc58482b501fe06287d
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Mon Aug 29 17:52:20 2022 +0200

    A == 0 ? A : -A    same as -A (when A is 0.0)
    
    The upcoming work for frange triggers a regression in
    gcc.dg/tree-ssa/phi-opt-24.c.
    
    For -O2 -fno-signed-zeros, we fail to transform the following into -A:
    
    float f0(float A)
    {
      //     A == 0? A : -A    same as -A
      if (A == 0)  return A;
      return -A;
    }
    
    This is because the abs/negative match.pd pattern here:
    
    /* abs/negative simplifications moved from fold_cond_expr_with_comparison,
       Need to handle (A - B) case as fold_cond_expr_with_comparison does.
       Need to handle UN* comparisons.
       ...
       ...
    
    Sees IL that has the 0.0 propagated.
    
    Instead of:
    
      <bb 2> [local count: 1073741824]:
      if (A_2(D) == 0.0)
        goto <bb 4>; [34.00%]
      else
        goto <bb 3>; [66.00%]
    
      <bb 3> [local count: 708669601]:
      _3 = -A_2(D);
    
      <bb 4> [local count: 1073741824]:
      # _1 = PHI <A_2(D)(2), _3(3)>
    
    It now sees:
    
      <bb 4> [local count: 1073741824]:
      # _1 = PHI <0.0(2), _3(3)>
    
    which it leaves untouched, causing the if conditional to survive.
    
    Changing integger_zerop to zerop fixes the problem.
    
    I did not include a testcase, as it's just phi-opt-24.c which will get
    triggered when I commit the frange with endpoints work.
    
    gcc/ChangeLog:
    
            * match.pd ((cmp @0 zerop) real_zerop (negate@1 @0)): Add variant
            for real zero.

Diff:
---
 gcc/match.pd | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/match.pd b/gcc/match.pd
index 1bb936fc401..f5fec634279 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -4804,7 +4804,7 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
     (if (!HONOR_SIGNED_ZEROS (type))
      @1))
   (simplify
-   (cnd (cmp @0 zerop) integer_zerop (negate@1 @0))
+   (cnd (cmp @0 zerop) zerop (negate@1 @0))
     (if (!HONOR_SIGNED_ZEROS (type))
      @1))
  )