[InstCombine] reduce FP-casted and bitcasted signbit check

PR45305:
https://bugs.llvm.org/show_bug.cgi?id=45305

Alive2 proofs:
http://volta.cs.utah.edu:8080/z/bVyrko
http://volta.cs.utah.edu:8080/z/Vxpz9q

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 63bf346..62b99e8 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2760,6 +2760,30 @@ static Instruction *foldICmpBitCast(ICmpInst &Cmp,
     if (match(BCSrcOp, m_UIToFP(m_Value(X))))
       if (Cmp.isEquality() && match(Op1, m_Zero()))
         return new ICmpInst(Pred, X, ConstantInt::getNullValue(X->getType()));
+
+    // If this is a sign-bit test of a bitcast of a casted FP value, eliminate
+    // the FP cast because the FP cast does not change the sign-bit.
+    const APInt *C;
+    bool TrueIfSigned;
+    if (match(Op1, m_APInt(C)) && Bitcast->hasOneUse() &&
+        isSignBitCheck(Pred, *C, TrueIfSigned)) {
+      if (match(BCSrcOp, m_FPExt(m_Value(X))) ||
+          match(BCSrcOp, m_FPTrunc(m_Value(X)))) {
+        // (bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) < 0
+        // (bitcast (fpext/fptrunc X)) to iX) > -1 --> (bitcast X to iY) > -1
+        Type *XType = X->getType();
+        Type *NewType = Builder.getIntNTy(XType->getScalarSizeInBits());
+        if (XType->isVectorTy())
+          NewType = VectorType::get(NewType, XType->getVectorNumElements());
+        Value *NewBitcast = Builder.CreateBitCast(X, NewType);
+        if (TrueIfSigned)
+          return new ICmpInst(ICmpInst::ICMP_SLT, NewBitcast,
+                              ConstantInt::getNullValue(NewType));
+        else
+          return new ICmpInst(ICmpInst::ICMP_SGT, NewBitcast,
+                              ConstantInt::getAllOnesValue(NewType));
+      }
+    }
   }
 
   // Test to see if the operands of the icmp are casted versions of other

diff --git a/llvm/test/Transforms/InstCombine/icmp.ll b/llvm/test/Transforms/InstCombine/icmp.ll
index b2c6213..41bf661 100644 (file)
--- a/llvm/test/Transforms/InstCombine/icmp.ll
+++ b/llvm/test/Transforms/InstCombine/icmp.ll
@@ -3609,9 +3609,8 @@ define <2 x i32> @Op1Negated_Vec(<2 x i32> %x) {
 
 define i1 @signbit_bitcast_fpext(float %x) {
 ; CHECK-LABEL: @signbit_bitcast_fpext(
-; CHECK-NEXT:    [[F:%.*]] = fpext float [[X:%.*]] to double
-; CHECK-NEXT:    [[B:%.*]] = bitcast double [[F]] to i64
-; CHECK-NEXT:    [[R:%.*]] = icmp slt i64 [[B]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float [[X:%.*]] to i32
+; CHECK-NEXT:    [[R:%.*]] = icmp slt i32 [[TMP1]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = fpext float %x to double
@@ -3622,9 +3621,8 @@ define i1 @signbit_bitcast_fpext(float %x) {
 
 define <2 x i1> @signbit_bitcast_fpext_vec(<2 x half> %x) {
 ; CHECK-LABEL: @signbit_bitcast_fpext_vec(
-; CHECK-NEXT:    [[F:%.*]] = fpext <2 x half> [[X:%.*]] to <2 x float>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x float> [[F]] to <2 x i32>
-; CHECK-NEXT:    [[R:%.*]] = icmp slt <2 x i32> [[B]], zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast <2 x half> [[X:%.*]] to <2 x i16>
+; CHECK-NEXT:    [[R:%.*]] = icmp slt <2 x i16> [[TMP1]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[R]]
 ;
   %f = fpext <2 x half> %x to <2 x float>
@@ -3635,9 +3633,8 @@ define <2 x i1> @signbit_bitcast_fpext_vec(<2 x half> %x) {
 
 define i1 @signbit_bitcast_fptrunc(float %x) {
 ; CHECK-LABEL: @signbit_bitcast_fptrunc(
-; CHECK-NEXT:    [[F:%.*]] = fptrunc float [[X:%.*]] to half
-; CHECK-NEXT:    [[B:%.*]] = bitcast half [[F]] to i16
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt i16 [[B]], -1
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float [[X:%.*]] to i32
+; CHECK-NEXT:    [[R:%.*]] = icmp sgt i32 [[TMP1]], -1
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = fptrunc float %x to half
@@ -3648,9 +3645,8 @@ define i1 @signbit_bitcast_fptrunc(float %x) {
 
 define <2 x i1> @signbit_bitcast_fptrunc_vec(<2 x double> %x) {
 ; CHECK-LABEL: @signbit_bitcast_fptrunc_vec(
-; CHECK-NEXT:    [[F:%.*]] = fptrunc <2 x double> [[X:%.*]] to <2 x half>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x half> [[F]] to <2 x i16>
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt <2 x i16> [[B]], <i16 -1, i16 -1>
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast <2 x double> [[X:%.*]] to <2 x i64>
+; CHECK-NEXT:    [[R:%.*]] = icmp sgt <2 x i64> [[TMP1]], <i64 -1, i64 -1>
 ; CHECK-NEXT:    ret <2 x i1> [[R]]
 ;
   %f = fptrunc <2 x double> %x to <2 x half>
@@ -3672,6 +3668,19 @@ define i1 @signbit_bitcast_fpext_wrong_cmp(float %x) {
   ret i1 %r
 }
 
+define <4 x i1> @signbit_bitcast_fpext_vec_wrong_bitcast(<2 x half> %x) {
+; CHECK-LABEL: @signbit_bitcast_fpext_vec_wrong_bitcast(
+; CHECK-NEXT:    [[F:%.*]] = fpext <2 x half> [[X:%.*]] to <2 x float>
+; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x float> [[F]] to <4 x i16>
+; CHECK-NEXT:    [[R:%.*]] = icmp sgt <4 x i16> [[B]], <i16 -1, i16 -1, i16 -1, i16 -1>
+; CHECK-NEXT:    ret <4 x i1> [[R]]
+;
+  %f = fpext <2 x half> %x to <2 x float>
+  %b = bitcast <2 x float> %f to <4 x i16>
+  %r = icmp sgt <4 x i16> %b, <i16 -1, i16 -1, i16 -1, i16 -1>
+  ret <4 x i1> %r
+}
+
 declare void @use_i64(i64)
 
 define i1 @signbit_bitcast_fpext_extra_use(float %x, i64* %p) {