[PATCH] [GVN} don't propagate equality comparisons of FP zero (PR22376)

[Sanjay Patel]

Hi pete, ArchDRobison, hfinkel,

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. As shown in http://llvm.org/bugs/show_bug.cgi?id=22376, we can miscompile with the current optimization.

I also checked to make sure that NaN comparisons are handled, and they always appear to be correctly optimized by SimplifyInstruction.

http://reviews.llvm.org/D7257

Files:
  lib/Transforms/Scalar/GVN.cpp
  test/Transforms/GVN/edge.ll

Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -2182,9 +2182,13 @@
 
       // 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.
+        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);
Index: test/Transforms/GVN/edge.ll
===================================================================
--- test/Transforms/GVN/edge.ll
+++ test/Transforms/GVN/edge.ll
@@ -69,11 +69,11 @@
   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 +86,30 @@
   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
 }

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