[PATCH] D28928: [ValueTracking] Implement SignBitMustBeZero correctly for sqrt.

[Justin Lebar via Phabricator via llvm-commits]

jlebar created this revision.

Previously we assumed that the result of sqrt(x) always had 0 as its
sign bit.  But sqrt(-0) == -0.


https://reviews.llvm.org/D28928

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll


Index: llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
+++ llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
@@ -104,6 +104,7 @@
 }
 
 declare float @llvm.fabs.f32(float)
+declare float @llvm.sqrt.f32(float)
 
 ; CHECK-LABEL: @fabs_select_positive_constants(
 ; CHECK: %select = select i1 %cmp, float 1.000000e+00, float 2.000000e+00
@@ -195,3 +196,13 @@
   %fabs = call float @llvm.fabs.f32(float %select)
   ret float %fabs
 }
+
+; CHECK-LABEL: @fabs_sqrt
+; CHECK: call float @llvm.sqrt.f32
+; CHECK: call float @llvm.fabs.f32
+define float @fabs_sqrt(float %a) {
+; The fabs can't be eliminated because llvm.sqrt.f32 may return -0.
+  %sqrt = call float @llvm.sqrt.f32(float %a)
+  %fabs = call float @llvm.fabs.f32(float %sqrt)
+  ret float %fabs
+}
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2651,7 +2651,8 @@
     return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
                                            Depth + 1);
   case Instruction::Call:
-    Intrinsic::ID IID = getIntrinsicForCallSite(cast<CallInst>(I), TLI);
+    const auto *CI = cast<CallInst>(I);
+    Intrinsic::ID IID = getIntrinsicForCallSite(CI, TLI);
     switch (IID) {
     default:
       break;
@@ -2668,12 +2669,18 @@
     case Intrinsic::exp:
     case Intrinsic::exp2:
     case Intrinsic::fabs:
-    case Intrinsic::sqrt:
       return true;
+
+    case Intrinsic::sqrt:
+      // sqrt(x) is always >= -0 or NaN.  Moreover, sqrt(x) == -0 iff x == -0.
+      // IEEE 754 section 6.3 lets us assume that the sign bit of the NaN
+      // returned here is whatever we want, so we assume that it's 0.
+      return !SignBitOnly || CannotBeNegativeZero(CI->getOperand(0), TLI);
+
     case Intrinsic::powi:
-      if (ConstantInt *CI = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      if (ConstantInt *Exponent = dyn_cast<ConstantInt>(I->getOperand(1))) {
         // powi(x,n) is non-negative if n is even.
-        if (CI->getBitWidth() <= 64 && CI->getSExtValue() % 2u == 0)
+        if (Exponent->getBitWidth() <= 64 && Exponent->getSExtValue() % 2u == 0)
           return true;
       }
       // TODO: This is not correct.  Given that exp is an integer, here are the


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