[InstCombine] bitrev (zext i1 X) --> select X, SMinC, 0

https://alive2.llvm.org/ce/z/ZXCtgi

This breaks the infinite combine loop for issue #59897,
but we may still need more changes to avoid those loops.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -1441,6 +1441,18 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
 
     break;
   }
+  case Intrinsic::bitreverse: {
+    // bitrev (zext i1 X to ?) --> X ? SignBitC : 0
+    Value *X;
+    if (match(II->getArgOperand(0), m_ZExt(m_Value(X))) &&
+        X->getType()->isIntOrIntVectorTy(1)) {
+      Type *Ty = II->getType();
+      APInt SignBit = APInt::getSignMask(Ty->getScalarSizeInBits());
+      return SelectInst::Create(X, ConstantInt::get(Ty, SignBit),
+                                ConstantInt::getNullValue(Ty));
+    }
+    break;
+  }
   case Intrinsic::bswap: {
     Value *IIOperand = II->getArgOperand(0);
 

llvm/test/Transforms/InstCombine/bitreverse.ll

@@ -323,7 +323,7 @@ define i32 @rev_i1(i1 %x) {
 ; CHECK-LABEL: @rev_i1(
 ; CHECK-NEXT:    [[Z:%.*]] = zext i1 [[X:%.*]] to i32
 ; CHECK-NEXT:    call void @use_i32(i32 [[Z]])
-; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.bitreverse.i32(i32 [[Z]])
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[X]], i32 -2147483648, i32 0
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %z = zext i1 %x to i32
@@ -334,8 +334,7 @@ define i32 @rev_i1(i1 %x) {
 
 define <2 x i8> @rev_v2i1(<2 x i1> %x) {
 ; CHECK-LABEL: @rev_v2i1(
-; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i1> [[X:%.*]] to <2 x i8>
-; CHECK-NEXT:    [[R:%.*]] = call <2 x i8> @llvm.bitreverse.v2i8(<2 x i8> [[Z]])
+; CHECK-NEXT:    [[R:%.*]] = select <2 x i1> [[X:%.*]], <2 x i8> <i8 -128, i8 -128>, <2 x i8> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %z = zext <2 x i1> %x to <2 x i8>
@@ -353,3 +352,19 @@ define i32 @rev_i2(i2 %x) {
   %r = call i32 @llvm.bitreverse.i32(i32 %z)
   ret i32 %r
 }
+
+; This used to infinite loop.
+
+define i64 @PR59897(i1 %X1_2) {
+; CHECK-LABEL: @PR59897(
+; CHECK-NEXT:    [[NOT_X1_2:%.*]] = xor i1 [[X1_2:%.*]], true
+; CHECK-NEXT:    [[X0_3X2X5X0:%.*]] = zext i1 [[NOT_X1_2]] to i64
+; CHECK-NEXT:    ret i64 [[X0_3X2X5X0]]
+;
+  %X1_3 = zext i1 %X1_2 to i32
+  %X8_3x2x2x0 = call i32 @llvm.bitreverse.i32(i32 %X1_3)
+  %X8_4x2x3x0 = xor i32 %X8_3x2x2x0, -1
+  %X0_3x2x4x0 = lshr i32 %X8_4x2x3x0, 31
+  %X0_3x2x5x0 = zext i32 %X0_3x2x4x0 to i64
+  ret i64 %X0_3x2x5x0
+}