[InstCombine] Simplify shift-by-sext to shift-by-zext

Summary:
This is valid for any `sext` bitwidth pair:
```
Processing /tmp/opt.ll..

----------------------------------------
  %signed = sext %y
  %r = shl %x, %signed
  ret %r
=>
  %unsigned = zext %y
  %r = shl %x, %unsigned
  ret %r
  %signed = sext %y

Done: 2016
Optimization is correct!
```

(This isn't so for funnel shifts, there it's illegal for e.g. i6->i7.)

Main motivation is the C++ semantics:
```
int shl(int a, char b) {
    return a << b;
}
```
ends as
```
  %3 = sext i8 %1 to i32
  %4 = shl i32 %0, %3
```
https://godbolt.org/z/0jgqUq
which is, as this shows, too pessimistic.

There is another problem here - we can only do the fold
if sext is one-use. But we can trivially have cases
where several shifts have the same sext shift amount.
This should be resolved, later.

Reviewers: spatel, nikic, RKSimon

Reviewed By: spatel

Subscribers: efriedma, hiraditya, nlopes, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68103

llvm-svn: 373106

3 files changed, 20 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index c88827f916c..ab1e484df93 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -241,6 +241,13 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   assert(Op0->getType() == Op1->getType());
 
+  // If the shift amount is a one-use `sext`, we can demote it to `zext`.
+  Value *Y;
+  if (match(Op1, m_OneUse(m_SExt(m_Value(Y))))) {
+    Value *NewExt = Builder.CreateZExt(Y, I.getType(), Op1->getName());
+    return BinaryOperator::Create(I.getOpcode(), Op0, NewExt);
+  }
+
   // See if we can fold away this shift.
   if (SimplifyDemandedInstructionBits(I))
     return &I;
diff --git a/llvm/test/Transforms/InstCombine/load-cmp.ll b/llvm/test/Transforms/InstCombine/load-cmp.ll
index ace1fb0ad26..7b24ce309a9 100644
--- a/llvm/test/Transforms/InstCombine/load-cmp.ll
+++ b/llvm/test/Transforms/InstCombine/load-cmp.ll
@@ -105,7 +105,7 @@ define i1 @test4(i32 %X) {
 
 define i1 @test4_i16(i16 %X) {
 ; CHECK-LABEL: @test4_i16(
-; CHECK-NEXT:    [[TMP1:%.*]] = sext i16 [[X:%.*]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i16 [[X:%.*]] to i32
 ; CHECK-NEXT:    [[TMP2:%.*]] = lshr i32 933, [[TMP1]]
 ; CHECK-NEXT:    [[TMP3:%.*]] = and i32 [[TMP2]], 1
 ; CHECK-NEXT:    [[R:%.*]] = icmp ne i32 [[TMP3]], 0
diff --git a/llvm/test/Transforms/InstCombine/shift-by-signext.ll b/llvm/test/Transforms/InstCombine/shift-by-signext.ll
index 9d6cf90dc19..b049dfac6f8 100644
--- a/llvm/test/Transforms/InstCombine/shift-by-signext.ll
+++ b/llvm/test/Transforms/InstCombine/shift-by-signext.ll
@@ -6,8 +6,8 @@
 
 define i32 @t0_shl(i32 %x, i8 %shamt) {
 ; CHECK-LABEL: @t0_shl(
-; CHECK-NEXT:    [[SHAMT_WIDE:%.*]] = sext i8 [[SHAMT:%.*]] to i32
-; CHECK-NEXT:    [[R:%.*]] = shl i32 [[X:%.*]], [[SHAMT_WIDE]]
+; CHECK-NEXT:    [[SHAMT_WIDE1:%.*]] = zext i8 [[SHAMT:%.*]] to i32
+; CHECK-NEXT:    [[R:%.*]] = shl i32 [[X:%.*]], [[SHAMT_WIDE1]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %shamt_wide = sext i8 %shamt to i32
@@ -16,8 +16,8 @@ define i32 @t0_shl(i32 %x, i8 %shamt) {
 }
 define i32 @t1_lshr(i32 %x, i8 %shamt) {
 ; CHECK-LABEL: @t1_lshr(
-; CHECK-NEXT:    [[SHAMT_WIDE:%.*]] = sext i8 [[SHAMT:%.*]] to i32
-; CHECK-NEXT:    [[R:%.*]] = lshr i32 [[X:%.*]], [[SHAMT_WIDE]]
+; CHECK-NEXT:    [[SHAMT_WIDE1:%.*]] = zext i8 [[SHAMT:%.*]] to i32
+; CHECK-NEXT:    [[R:%.*]] = lshr i32 [[X:%.*]], [[SHAMT_WIDE1]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %shamt_wide = sext i8 %shamt to i32
@@ -26,8 +26,8 @@ define i32 @t1_lshr(i32 %x, i8 %shamt) {
 }
 define i32 @t2_ashr(i32 %x, i8 %shamt) {
 ; CHECK-LABEL: @t2_ashr(
-; CHECK-NEXT:    [[SHAMT_WIDE:%.*]] = sext i8 [[SHAMT:%.*]] to i32
-; CHECK-NEXT:    [[R:%.*]] = ashr i32 [[X:%.*]], [[SHAMT_WIDE]]
+; CHECK-NEXT:    [[SHAMT_WIDE1:%.*]] = zext i8 [[SHAMT:%.*]] to i32
+; CHECK-NEXT:    [[R:%.*]] = ashr i32 [[X:%.*]], [[SHAMT_WIDE1]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %shamt_wide = sext i8 %shamt to i32
@@ -37,8 +37,8 @@ define i32 @t2_ashr(i32 %x, i8 %shamt) {
 
 define <2 x i32> @t3_vec_shl(<2 x i32> %x, <2 x i8> %shamt) {
 ; CHECK-LABEL: @t3_vec_shl(
-; CHECK-NEXT:    [[SHAMT_WIDE:%.*]] = sext <2 x i8> [[SHAMT:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[R:%.*]] = shl <2 x i32> [[X:%.*]], [[SHAMT_WIDE]]
+; CHECK-NEXT:    [[SHAMT_WIDE1:%.*]] = zext <2 x i8> [[SHAMT:%.*]] to <2 x i32>
+; CHECK-NEXT:    [[R:%.*]] = shl <2 x i32> [[X:%.*]], [[SHAMT_WIDE1]]
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
   %shamt_wide = sext <2 x i8> %shamt to <2 x i32>
@@ -47,8 +47,8 @@ define <2 x i32> @t3_vec_shl(<2 x i32> %x, <2 x i8> %shamt) {
 }
 define <2 x i32> @t4_vec_lshr(<2 x i32> %x, <2 x i8> %shamt) {
 ; CHECK-LABEL: @t4_vec_lshr(
-; CHECK-NEXT:    [[SHAMT_WIDE:%.*]] = sext <2 x i8> [[SHAMT:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[R:%.*]] = lshr <2 x i32> [[X:%.*]], [[SHAMT_WIDE]]
+; CHECK-NEXT:    [[SHAMT_WIDE1:%.*]] = zext <2 x i8> [[SHAMT:%.*]] to <2 x i32>
+; CHECK-NEXT:    [[R:%.*]] = lshr <2 x i32> [[X:%.*]], [[SHAMT_WIDE1]]
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
   %shamt_wide = sext <2 x i8> %shamt to <2 x i32>
@@ -57,8 +57,8 @@ define <2 x i32> @t4_vec_lshr(<2 x i32> %x, <2 x i8> %shamt) {
 }
 define <2 x i32> @t5_vec_ashr(<2 x i32> %x, <2 x i8> %shamt) {
 ; CHECK-LABEL: @t5_vec_ashr(
-; CHECK-NEXT:    [[SHAMT_WIDE:%.*]] = sext <2 x i8> [[SHAMT:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[R:%.*]] = ashr <2 x i32> [[X:%.*]], [[SHAMT_WIDE]]
+; CHECK-NEXT:    [[SHAMT_WIDE1:%.*]] = zext <2 x i8> [[SHAMT:%.*]] to <2 x i32>
+; CHECK-NEXT:    [[R:%.*]] = ashr <2 x i32> [[X:%.*]], [[SHAMT_WIDE1]]
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
   %shamt_wide = sext <2 x i8> %shamt to <2 x i32>