[ValueTracking] Teach cannotBeOrderedLessThanZeroImpl to look through ExtractElement.

[craig.topper]

Summary

This is similar to what's done in computeKnownBits and computeSignBits. Don't do anything fancy just collect information valid for any element.

[spatel]

LGTM.

llvm/trunk/lib/Analysis/ValueTracking.cpp

return cannotBeOrderedLessThanZeroImpl(I->getOperand(1), TLI, SignBitOnly,
                                            Depth + 1) &&
            cannotBeOrderedLessThanZeroImpl(I->getOperand(2), TLI, SignBitOnly,
                                            Depth + 1);
   case Instruction::FPExt:
   case Instruction::FPTrunc:
     // Widening/narrowing never change sign.
     return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
                                            Depth + 1);
+  case Instruction::ExtractElement:
+    // Look through extract element. At the moment we keep this simple and skip
+    // tracking the specific element. But at least we might find information
+    // valid for all elements of the vector.
+    return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
+                                           Depth + 1);
   case Instruction::Call:
     const auto *CI = cast<CallInst>(I);
     Intrinsic::ID IID = getIntrinsicForCallSite(CI, TLI);
     switch (IID) {
     default:
       break;
     case Intrinsic::maxnum:
       return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,

llvm/trunk/test/Transforms/InstSimplify/floating-point-arithmetic.ll

 ; CHECK-NEXT:    [[SQRT:%.*]] = call nnan float @llvm.sqrt.f32(float [[B]])
 ; CHECK-NEXT:    ret float [[SQRT]]
 ;
   %b = call float @llvm.fabs.f32(float %a)
   %sqrt = call nnan float @llvm.sqrt.f32(float %b)
   %fabs = call float @llvm.fabs.f32(float %sqrt)
   ret float %fabs
 }
+
+define float @fabs_select_positive_constants_vector_extract(i32 %c) {
+; CHECK-LABEL: @fabs_select_positive_constants_vector_extract(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[C:%.*]], 0
+; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[CMP]], <2 x float> <float 1.000000e+00, float 1.000000e+00>, <2 x float> <float 2.000000e+00, float 2.000000e+00>
+; CHECK-NEXT:    [[EXTRACT:%.*]] = extractelement <2 x float> [[SELECT]], i32 0
+; CHECK-NEXT:    ret float [[EXTRACT]]
+;
+  %cmp = icmp eq i32 %c, 0
+  %select = select i1 %cmp, <2 x float> <float 1.0, float 1.0>, <2 x float> <float 2.0, float 2.0>
+  %extract = extractelement <2 x float> %select, i32 0
+  %fabs = call float @llvm.fabs.f32(float %extract)
+  ret float %fabs
+}