[llvm] 5819f4a - [InstCombine] C0 <<{nsw, nuw} (X - C1) --> (C0 >> C1) << X

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2022-04-21T11:38:27-04:00
New Revision: 5819f4a422865fc9a8ea4dc772769e14010ff6a7

URL: https://github.com/llvm/llvm-project/commit/5819f4a422865fc9a8ea4dc772769e14010ff6a7
DIFF: https://github.com/llvm/llvm-project/commit/5819f4a422865fc9a8ea4dc772769e14010ff6a7.diff

LOG: [InstCombine] C0 <<{nsw, nuw} (X - C1) --> (C0 >> C1) << X

This is similar to an existing pre-shift-of-constant fold:
8a9c70fc01e6
...but in this case, we need no-wrap on the shl and a negative
offset:
https://alive2.llvm.org/ce/z/_RVz99

Fixes #54890

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
    llvm/test/Transforms/InstCombine/shift-add.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 89558ee06745c..73684495e8bf3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -988,6 +988,23 @@ Instruction *InstCombinerImpl::visitShl(BinaryOperator &I) {
     return BinaryOperator::CreateLShr(
         ConstantInt::get(Ty, APInt::getSignMask(BitWidth)), X);
 
+  // Try to pre-shift a constant shifted by a variable amount:
+  // C << (X + AddC) --> (C >> -AddC) << X
+  // This requires a no-wrap flag and negative offset constant.
+  const APInt *AddC;
+  if ((I.hasNoSignedWrap() || I.hasNoUnsignedWrap()) &&
+      match(Op0, m_APInt(C)) &&
+      match(Op1, m_Add(m_Value(X), m_APInt(AddC))) && AddC->isNegative()) {
+    assert(!C->isZero() && "Expected simplify of shifted zero");
+    unsigned PosOffset = (-*AddC).getLimitedValue();
+    if (C->eq(C->lshr(PosOffset).shl(PosOffset))) {
+      Constant *NewC = ConstantInt::get(Ty, C->lshr(PosOffset));
+      Instruction *NewShl = BinaryOperator::CreateShl(NewC, X);
+      NewShl->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
+      return NewShl;
+    }
+  }
+
   return nullptr;
 }
 

diff  --git a/llvm/test/Transforms/InstCombine/shift-add.ll b/llvm/test/Transforms/InstCombine/shift-add.ll
index 141551978377e..6cf5451274185 100644
--- a/llvm/test/Transforms/InstCombine/shift-add.ll
+++ b/llvm/test/Transforms/InstCombine/shift-add.ll
@@ -174,8 +174,7 @@ define i32 @shl_add_nsw(i32 %x) {
 
 define i32 @shl_nsw_add_negative(i32 %x) {
 ; CHECK-LABEL: @shl_nsw_add_negative(
-; CHECK-NEXT:    [[A:%.*]] = add i32 [[X:%.*]], -1
-; CHECK-NEXT:    [[R:%.*]] = shl nsw i32 2, [[A]]
+; CHECK-NEXT:    [[R:%.*]] = shl i32 1, [[X:%.*]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %a = add i32 %x, -1
@@ -183,11 +182,14 @@ define i32 @shl_nsw_add_negative(i32 %x) {
   ret i32 %r
 }
 
+; vectors and extra uses are allowed
+; nuw propagates to the new shift
+
 define <2 x i8> @shl_nuw_add_negative_splat_uses(<2 x i8> %x, <2 x i8>* %p) {
 ; CHECK-LABEL: @shl_nuw_add_negative_splat_uses(
 ; CHECK-NEXT:    [[A:%.*]] = add <2 x i8> [[X:%.*]], <i8 -2, i8 -2>
 ; CHECK-NEXT:    store <2 x i8> [[A]], <2 x i8>* [[P:%.*]], align 2
-; CHECK-NEXT:    [[R:%.*]] = shl nuw <2 x i8> <i8 12, i8 12>, [[A]]
+; CHECK-NEXT:    [[R:%.*]] = shl nuw <2 x i8> <i8 3, i8 3>, [[X]]
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %a = add <2 x i8> %x, <i8 -2, i8 -2>
@@ -196,6 +198,8 @@ define <2 x i8> @shl_nuw_add_negative_splat_uses(<2 x i8> %x, <2 x i8>* %p) {
   ret <2 x i8> %r
 }
 
+; negative test - shift constant must have enough trailing zeros to allow the pre-shift
+
 define i32 @shl_nsw_add_negative_invalid_constant(i32 %x) {
 ; CHECK-LABEL: @shl_nsw_add_negative_invalid_constant(
 ; CHECK-NEXT:    [[A:%.*]] = add i32 [[X:%.*]], -2
@@ -207,6 +211,8 @@ define i32 @shl_nsw_add_negative_invalid_constant(i32 %x) {
   ret i32 %r
 }
 
+; negative test - the offset constant must be negative
+
 define i32 @shl_nsw_add_positive_invalid_constant(i32 %x) {
 ; CHECK-LABEL: @shl_nsw_add_positive_invalid_constant(
 ; CHECK-NEXT:    [[A:%.*]] = add i32 [[X:%.*]], 2