[PATCH] D36443: [InstCombine] Support pulling left shifts through a subtract with constant LHS

[Craig Topper via Phabricator via llvm-commits]

craig.topper updated this revision to Diff 110256.
craig.topper added a comment.

Add a simpler test case and a more mathematical comment.

Not sure how to preserve NSW/NUW without calling computeKnownBits. Any ideas?


https://reviews.llvm.org/D36443

Files:
  lib/Transforms/InstCombine/InstCombineShifts.cpp
  test/Transforms/InstCombine/sub.ll


Index: test/Transforms/InstCombine/sub.ll
===================================================================
--- test/Transforms/InstCombine/sub.ll
+++ test/Transforms/InstCombine/sub.ll
@@ -1070,3 +1070,25 @@
   store i8* %gep2, i8** @dummy_global2
   ret i64 %sub
 }
+
+define i32 @test62(i32 %A) {
+; CHECK-LABEL: @test62(
+; CHECK-NEXT:    [[B:%.*]] = shl i32 [[A:%.*]], 1
+; CHECK-NEXT:    [[C:%.*]] = sub i32 2, [[B]]
+; CHECK-NEXT:    ret i32 [[C]]
+;
+  %B = sub i32 1, %A
+  %C = shl i32 %B, 1
+  ret i32 %C
+}
+
+define i32 @test63(i32 %A) {
+; CHECK-LABEL: @test63(
+; CHECK-NEXT:    [[B:%.*]] = shl i32 [[A:%.*]], 1
+; CHECK-NEXT:    ret i32 [[B]]
+;
+  %B = sub i32 1, %A
+  %C = shl i32 %B, 1
+  %D = sub i32 2, %C
+  ret i32 %D
+}
Index: lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -510,6 +510,20 @@
                                         NewRHS);
         }
       }
+
+      // If the operand is a subtract with a constant LHS, and the shift
+      // is the only use, we can pull it out of the shift.
+      // This folds (shl (sub C1, X), C2) -> (sub (C1 << C2), (shl X, C2))
+      if (isLeftShift && Op0BO->getOpcode() == Instruction::Sub &&
+          match(Op0BO->getOperand(0), m_APInt(Op0C))) {
+        Constant *NewRHS = ConstantExpr::get(I.getOpcode(),
+                                   cast<Constant>(Op0BO->getOperand(0)), Op1);
+
+        Value *NewShift = Builder.CreateShl(Op0BO->getOperand(1), Op1);
+        NewShift->takeName(Op0BO);
+
+        return BinaryOperator::CreateSub(NewRHS, NewShift);
+      }
     }
   }
 


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