[llvm] r227491 - [GVN] don't propagate equality comparisons of FP zero (PR22376)

[Sanjay Patel]

Author: spatel
Date: Thu Jan 29 14:51:49 2015
New Revision: 227491

URL: http://llvm.org/viewvc/llvm-project?rev=227491&view=rev
Log:
[GVN] don't propagate equality comparisons of FP zero (PR22376)

In http://reviews.llvm.org/D6911, we allowed GVN to propagate FP equalities
to allow some simple value range optimizations. But that introduced a bug
when comparing to -0.0 or 0.0: these compare equal even though they are not
bitwise identical.

This patch disallows propagating zero constants in equality comparisons. 
Fixes: http://llvm.org/bugs/show_bug.cgi?id=22376

Differential Revision: http://reviews.llvm.org/D7257


Modified:
    llvm/trunk/lib/Transforms/Scalar/GVN.cpp
    llvm/trunk/test/Transforms/GVN/edge.ll

Modified: llvm/trunk/lib/Transforms/Scalar/GVN.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/GVN.cpp?rev=227491&r1=227490&r2=227491&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/GVN.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/GVN.cpp Thu Jan 29 14:51:49 2015
@@ -2182,9 +2182,16 @@ bool GVN::propagateEquality(Value *LHS,
 
       // Handle the floating point versions of equality comparisons too.
       if ((isKnownTrue && Cmp->getPredicate() == CmpInst::FCMP_OEQ) ||
-          (isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE))
-        Worklist.push_back(std::make_pair(Op0, Op1));
-
+          (isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE)) {
+        // Floating point -0.0 and 0.0 compare equal, so we can't
+        // propagate a constant based on that comparison.
+        // FIXME: We should do this optimization if 'no signed zeros' is
+        // applicable via an instruction-level fast-math-flag or some other
+        // indicator that relaxed FP semantics are being used.
+        if (!isa<ConstantFP>(Op1) || !cast<ConstantFP>(Op1)->isZero())
+          Worklist.push_back(std::make_pair(Op0, Op1));
+      }
+ 
       // If "A >= B" is known true, replace "A < B" with false everywhere.
       CmpInst::Predicate NotPred = Cmp->getInversePredicate();
       Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse);

Modified: llvm/trunk/test/Transforms/GVN/edge.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/GVN/edge.ll?rev=227491&r1=227490&r2=227491&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/GVN/edge.ll (original)
+++ llvm/trunk/test/Transforms/GVN/edge.ll Thu Jan 29 14:51:49 2015
@@ -69,11 +69,11 @@ if:
   br label %return
 
 return:
-  %retval.0 = phi double [ %div, %if ], [ %x, %entry ]
-  ret double %retval.0
+  %retval = phi double [ %div, %if ], [ %x, %entry ]
+  ret double %retval
 
 ; CHECK-LABEL: define double @fcmp_oeq(
-; CHECK: %div = fdiv double %x, 2.000000e+00
+; CHECK: %div = fdiv double %x, 2.0
 }
 
 define double @fcmp_une(double %x, double %y) {
@@ -86,10 +86,46 @@ else:
   br label %return
 
 return:
-  %retval.0 = phi double [ %div, %else ], [ %x, %entry ]
-  ret double %retval.0
+  %retval = phi double [ %div, %else ], [ %x, %entry ]
+  ret double %retval
 
 ; CHECK-LABEL: define double @fcmp_une(
-; CHECK: %div = fdiv double %x, 2.000000e+00
+; CHECK: %div = fdiv double %x, 2.0
 }
 
+; PR22376 - We can't propagate zero constants because -0.0 
+; compares equal to 0.0. If %y is -0.0 in this test case,
+; we would produce the wrong sign on the infinity return value.
+define double @fcmp_oeq_zero(double %x, double %y) {
+entry:
+  %cmp = fcmp oeq double %y, 0.0
+  br i1 %cmp, label %if, label %return
+
+if:
+  %div = fdiv double %x, %y
+  br label %return
+
+return:
+  %retval = phi double [ %div, %if ], [ %x, %entry ]
+  ret double %retval
+
+; CHECK-LABEL: define double @fcmp_oeq_zero(
+; CHECK: %div = fdiv double %x, %y
+}
+
+define double @fcmp_une_zero(double %x, double %y) {
+entry:
+  %cmp = fcmp une double %y, -0.0
+  br i1 %cmp, label %return, label %else
+
+else:
+  %div = fdiv double %x, %y
+  br label %return
+
+return:
+  %retval = phi double [ %div, %else ], [ %x, %entry ]
+  ret double %retval
+
+; CHECK-LABEL: define double @fcmp_une_zero(
+; CHECK: %div = fdiv double %x, %y
+}