From 364359e4fc0a0673574a8b4e8a04039abbe9ce5e Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper@intel.com>
Date: Tue, 8 Aug 2017 20:14:11 +0000
Subject: [PATCH] [InstCombine] Support pulling left shifts through a subtract
 with constant LHS

We already support pulling through an add with constant RHS. We can do the same for subtract.

Differential Revision: https://reviews.llvm.org/D36443

llvm-svn: 310407
---
 .../Transforms/InstCombine/InstCombineShifts.cpp   | 14 +++++++
 llvm/test/Transforms/InstCombine/sub.ll            | 44 ++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 5f21666..098079a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -510,6 +510,20 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
                                         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);
+      }
     }
   }
 
diff --git a/llvm/test/Transforms/InstCombine/sub.ll b/llvm/test/Transforms/InstCombine/sub.ll
index d20668f..5d6d5be 100644
--- a/llvm/test/Transforms/InstCombine/sub.ll
+++ b/llvm/test/Transforms/InstCombine/sub.ll
@@ -1070,3 +1070,47 @@ define i64 @test61([100 x [100 x i8]]* %foo, i64 %i, i64 %j) {
   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 <2 x i32> @test62vec(<2 x i32> %A) {
+; CHECK-LABEL: @test62vec(
+; CHECK-NEXT:    [[B:%.*]] = shl <2 x i32> [[A:%.*]], <i32 1, i32 1>
+; CHECK-NEXT:    [[C:%.*]] = sub <2 x i32> <i32 2, i32 2>, [[B]]
+; CHECK-NEXT:    ret <2 x i32> [[C]]
+;
+  %B = sub <2 x i32> <i32 1, i32 1>, %A
+  %C = shl <2 x i32> %B, <i32 1, i32 1>
+  ret <2 x 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
+}
+
+define <2 x i32> @test63vec(<2 x i32> %A) {
+; CHECK-LABEL: @test63vec(
+; CHECK-NEXT:    [[B:%.*]] = shl <2 x i32> [[A:%.*]], <i32 1, i32 1>
+; CHECK-NEXT:    ret <2 x i32> [[B]]
+;
+  %B = sub <2 x i32> <i32 1, i32 1>, %A
+  %C = shl <2 x i32> %B, <i32 1, i32 1>
+  %D = sub <2 x i32> <i32 2, i32 2>, %C
+  ret <2 x i32> %D
+}
-- 
2.7.4