[InstCombine] (1 << (C - x)) -> ((1 << C) >> x) if C is bitwidth - 1

Summary:
```
%a = sub i32 31, %x
%r = shl i32 1, %a
  =>
%d = shl i32 1, 31
%r = lshr i32 %d, %x

Done: 1
Optimization is correct!
```

https://rise4fun.com/Alive/btZm

Reviewers: spatel, lebedev.ri, nikic

Reviewed By: lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63652

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364073 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 21f1ff56602b6327e33dee2a73fc5844d24d09a2..655f40a0de057289b6afb889cfb3a3f30e98c37e 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -582,6 +582,8 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
 
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   Type *Ty = I.getType();
+  unsigned BitWidth = Ty->getScalarSizeInBits();
+
   const APInt *ShAmtAPInt;
   if (match(Op1, m_APInt(ShAmtAPInt))) {
     unsigned ShAmt = ShAmtAPInt->getZExtValue();
@@ -670,6 +672,12 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
       return BinaryOperator::CreateMul(X, ConstantExpr::getShl(C2, C1));
   }
 
+  // (1 << (C - x)) -> ((1 << C) >> x) if C is bitwidth - 1
+  if (match(Op0, m_One()) &&
+      match(Op1, m_Sub(m_SpecificInt(BitWidth - 1), m_Value(X))))
+    return BinaryOperator::CreateLShr(
+        ConstantInt::get(Ty, APInt::getSignMask(BitWidth)), X);
+
   return nullptr;
 }
 

diff --git a/test/Transforms/InstCombine/shl-sub.ll b/test/Transforms/InstCombine/shl-sub.ll
index 43ba548e92f57b8ba78ab19c8f663aab2b02e743..fc2939e66508552c5671640cbc45f20e4151d001 100644 (file)
--- a/test/Transforms/InstCombine/shl-sub.ll
+++ b/test/Transforms/InstCombine/shl-sub.ll
@@ -3,8 +3,7 @@
 
 define i32 @shl_sub_i32(i32 %x) {
 ; CHECK-LABEL: @shl_sub_i32(
-; CHECK-NEXT:    [[S:%.*]] = sub i32 31, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = shl i32 1, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = lshr i32 -2147483648, [[X:%.*]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %s = sub i32 31, %x
@@ -16,7 +15,7 @@ define i32 @shl_sub_multiuse_i32(i32 %x) {
 ; CHECK-LABEL: @shl_sub_multiuse_i32(
 ; CHECK-NEXT:    [[S:%.*]] = sub i32 31, [[X:%.*]]
 ; CHECK-NEXT:    call void @use(i32 [[S]])
-; CHECK-NEXT:    [[R:%.*]] = shl i32 1, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = lshr i32 -2147483648, [[X]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %s = sub i32 31, %x
@@ -27,8 +26,7 @@ define i32 @shl_sub_multiuse_i32(i32 %x) {
 
 define i8 @shl_sub_i8(i8 %x) {
 ; CHECK-LABEL: @shl_sub_i8(
-; CHECK-NEXT:    [[S:%.*]] = sub i8 7, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = shl i8 1, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = lshr i8 -128, [[X:%.*]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %s = sub i8 7, %x
@@ -38,8 +36,7 @@ define i8 @shl_sub_i8(i8 %x) {
 
 define i64 @shl_sub_i64(i64 %x) {
 ; CHECK-LABEL: @shl_sub_i64(
-; CHECK-NEXT:    [[S:%.*]] = sub i64 63, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = shl i64 1, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = lshr i64 -9223372036854775808, [[X:%.*]]
 ; CHECK-NEXT:    ret i64 [[R]]
 ;
   %s = sub i64 63, %x
@@ -49,8 +46,7 @@ define i64 @shl_sub_i64(i64 %x) {
 
 define <2 x i64> @shl_sub_i64_vec(<2 x i64> %x) {
 ; CHECK-LABEL: @shl_sub_i64_vec(
-; CHECK-NEXT:    [[S:%.*]] = sub <2 x i64> <i64 63, i64 63>, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = shl <2 x i64> <i64 1, i64 1>, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = lshr <2 x i64> <i64 -9223372036854775808, i64 -9223372036854775808>, [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x i64> [[R]]
 ;
   %s = sub <2 x i64> <i64 63, i64 63>, %x