[InstCombine] Inefficient pattern for high-bits checking (PR38708)

Summary:
It is sometimes important to check that some newly-computed value
is non-negative and only `n` bits wide (where `n` is a variable.)
There are **many** ways to check that:
https://godbolt.org/z/o4RB8D
The last variant seems best?
(I'm sure there are some other variations i haven't thought of..)

Let's handle the second variant first, since it is much simpler.
https://rise4fun.com/Alive/LYjY

https://bugs.llvm.org/show_bug.cgi?id=38708

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51985

llvm-svn: 342067

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -4623,6 +4623,41 @@ static Instruction *canonicalizeICmpBool(ICmpInst &I,
   }
 }
 
+// Transform pattern like:
+//   (1 << Y) u<= X
+//   (1 << Y) u>  X
+// Into:
+//   (X l>> Y) != 0
+//   (X l>> Y) == 0
+static Instruction *foldICmpWithHighBitMask(ICmpInst &Cmp,
+                                            InstCombiner::BuilderTy &Builder) {
+  ICmpInst::Predicate Pred;
+  Value *X, *Y;
+  if (!match(&Cmp,
+             m_c_ICmp(Pred, m_OneUse(m_Shl(m_One(), m_Value(Y))), m_Value(X))))
+    return nullptr;
+
+  // We want X to be the icmp's second operand, so swap predicate if it is not.
+  if (Cmp.getOperand(0) == X)
+    Pred = Cmp.getSwappedPredicate();
+
+  ICmpInst::Predicate NewPred;
+  switch (Pred) {
+  case ICmpInst::ICMP_ULE:
+    NewPred = ICmpInst::ICMP_NE;
+    break;
+  case ICmpInst::ICMP_UGT:
+    NewPred = ICmpInst::ICMP_EQ;
+    break;
+  default:
+    return nullptr;
+  }
+
+  Value *NewX = Builder.CreateLShr(X, Y, X->getName() + ".highbits");
+  Constant *Zero = Constant::getNullValue(NewX->getType());
+  return CmpInst::Create(Instruction::ICmp, NewPred, NewX, Zero);
+}
+
 static Instruction *foldVectorCmp(CmpInst &Cmp,
                                   InstCombiner::BuilderTy &Builder) {
   // If both arguments of the cmp are shuffles that use the same mask and
@@ -4913,6 +4948,9 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
       return foldICmpAddOpConst(X, *C, I.getSwappedPredicate());
   }
 
+  if (Instruction *Res = foldICmpWithHighBitMask(I, Builder))
+    return Res;
+
   if (I.getType()->isVectorTy())
     if (Instruction *Res = foldVectorCmp(I, Builder))
       return Res;

llvm/test/Transforms/InstCombine/icmp-ugt-of-shl-1-by-bits-and-val-to-icmp-eq-of-lshr-val-by-bits-and-0.ll

@@ -14,8 +14,8 @@
 
 define i1 @p0(i8 %val, i8 %bits) {
 ; CHECK-LABEL: @p0(
-; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ugt i8 [[T0]], [[VAL:%.*]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr i8 [[VAL:%.*]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[VAL_HIGHBITS]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %t0 = shl i8 1, %bits
@@ -29,8 +29,8 @@ define i1 @p0(i8 %val, i8 %bits) {
 
 define <2 x i1> @p1_vec(<2 x i8> %val, <2 x i8> %bits) {
 ; CHECK-LABEL: @p1_vec(
-; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 1>, [[BITS:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ugt <2 x i8> [[T0]], [[VAL:%.*]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr <2 x i8> [[VAL:%.*]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp eq <2 x i8> [[VAL_HIGHBITS]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[R]]
 ;
   %t0 = shl <2 x i8> <i8 1, i8 1>, %bits
@@ -40,8 +40,8 @@ define <2 x i1> @p1_vec(<2 x i8> %val, <2 x i8> %bits) {
 
 define <3 x i1> @p2_vec_undef(<3 x i8> %val, <3 x i8> %bits) {
 ; CHECK-LABEL: @p2_vec_undef(
-; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 undef, i8 1>, [[BITS:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ugt <3 x i8> [[T0]], [[VAL:%.*]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr <3 x i8> [[VAL:%.*]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp eq <3 x i8> [[VAL_HIGHBITS]], zeroinitializer
 ; CHECK-NEXT:    ret <3 x i1> [[R]]
 ;
   %t0 = shl <3 x i8> <i8 1, i8 undef, i8 1>, %bits
@@ -57,9 +57,9 @@ declare i8 @gen8()
 
 define i1 @c0(i8 %bits) {
 ; CHECK-LABEL: @c0(
-; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
 ; CHECK-NEXT:    [[VAL:%.*]] = call i8 @gen8()
-; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[VAL]], [[T0]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr i8 [[VAL]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[VAL_HIGHBITS]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %t0 = shl i8 1, %bits

llvm/test/Transforms/InstCombine/icmp-ule-of-shl-1-by-bits-and-val-to-icmp-ne-of-lshr-val-by-bits-and-0.ll

@@ -14,8 +14,8 @@
 
 define i1 @p0(i8 %val, i8 %bits) {
 ; CHECK-LABEL: @p0(
-; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ule i8 [[T0]], [[VAL:%.*]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr i8 [[VAL:%.*]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[VAL_HIGHBITS]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %t0 = shl i8 1, %bits
@@ -29,8 +29,8 @@ define i1 @p0(i8 %val, i8 %bits) {
 
 define <2 x i1> @p1_vec(<2 x i8> %val, <2 x i8> %bits) {
 ; CHECK-LABEL: @p1_vec(
-; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 1>, [[BITS:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ule <2 x i8> [[T0]], [[VAL:%.*]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr <2 x i8> [[VAL:%.*]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne <2 x i8> [[VAL_HIGHBITS]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[R]]
 ;
   %t0 = shl <2 x i8> <i8 1, i8 1>, %bits
@@ -40,8 +40,8 @@ define <2 x i1> @p1_vec(<2 x i8> %val, <2 x i8> %bits) {
 
 define <3 x i1> @p2_vec_undef(<3 x i8> %val, <3 x i8> %bits) {
 ; CHECK-LABEL: @p2_vec_undef(
-; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 undef, i8 1>, [[BITS:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ule <3 x i8> [[T0]], [[VAL:%.*]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr <3 x i8> [[VAL:%.*]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne <3 x i8> [[VAL_HIGHBITS]], zeroinitializer
 ; CHECK-NEXT:    ret <3 x i1> [[R]]
 ;
   %t0 = shl <3 x i8> <i8 1, i8 undef, i8 1>, %bits
@@ -57,9 +57,9 @@ declare i8 @gen8()
 
 define i1 @c0(i8 %bits) {
 ; CHECK-LABEL: @c0(
-; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
 ; CHECK-NEXT:    [[VAL:%.*]] = call i8 @gen8()
-; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[VAL]], [[T0]]
+; CHECK-NEXT:    [[VAL_HIGHBITS:%.*]] = lshr i8 [[VAL]], [[BITS:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[VAL_HIGHBITS]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %t0 = shl i8 1, %bits