[llvm] r334371 - [InstCombine] Fold (x << y) >> y -> x & (-1 >> y)

[Roman Lebedev via llvm-commits]

Author: lebedevri
Date: Sun Jun 10 13:10:06 2018
New Revision: 334371

URL: http://llvm.org/viewvc/llvm-project?rev=334371&view=rev
Log:
[InstCombine] Fold (x << y) >> y  ->  x & (-1 >> y)

Summary:
We already do it for splat constants, but not just values.
Also, undef cases are mostly non-functional.

https://bugs.llvm.org/show_bug.cgi?id=37603
https://rise4fun.com/Alive/cplX

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47980

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
    llvm/trunk/test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp?rev=334371&r1=334370&r2=334371&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp Sun Jun 10 13:10:06 2018
@@ -802,6 +802,15 @@ Instruction *InstCombiner::visitLShr(Bin
       return &I;
     }
   }
+
+  // Transform  (x << y) >> y  to  x & (-1 >> y)
+  Value *X;
+  if (match(Op0, m_OneUse(m_Shl(m_Value(X), m_Specific(Op1))))) {
+    Constant *AllOnes = ConstantInt::getAllOnesValue(Ty);
+    Value *Mask = Builder.CreateLShr(AllOnes, Op1);
+    return BinaryOperator::CreateAnd(Mask, X);
+  }
+
   return nullptr;
 }
 

Modified: llvm/trunk/test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll?rev=334371&r1=334370&r2=334371&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll Sun Jun 10 13:10:06 2018
@@ -14,8 +14,8 @@
 
 define i32 @positive_samevar(i32 %x, i32 %y) {
 ; CHECK-LABEL: @positive_samevar(
-; CHECK-NEXT:    [[TMP0:%.*]] = shl i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[RET:%.*]] = lshr i32 [[TMP0]], [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 -1, [[Y:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = and i32 [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret i32 [[RET]]
 ;
   %tmp0 = shl i32 %x, %y
@@ -124,8 +124,8 @@ define i32 @positive_biggerLshr_shlnuw_l
 
 define <2 x i32> @positive_samevar_vec(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @positive_samevar_vec(
-; CHECK-NEXT:    [[TMP0:%.*]] = shl <2 x i32> [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[RET:%.*]] = lshr <2 x i32> [[TMP0]], [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> <i32 -1, i32 -1>, [[Y:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = and <2 x i32> [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x i32> [[RET]]
 ;
   %tmp0 = shl <2 x i32> %x, %y
@@ -171,8 +171,7 @@ define <3 x i32> @positive_sameconst_vec
 
 define <3 x i32> @positive_sameconst_vec_undef2(<3 x i32> %x) {
 ; CHECK-LABEL: @positive_sameconst_vec_undef2(
-; CHECK-NEXT:    [[TMP0:%.*]] = shl <3 x i32> [[X:%.*]], <i32 5, i32 undef, i32 5>
-; CHECK-NEXT:    [[RET:%.*]] = lshr <3 x i32> [[TMP0]], <i32 5, i32 undef, i32 5>
+; CHECK-NEXT:    [[RET:%.*]] = and <3 x i32> [[X:%.*]], <i32 134217727, i32 undef, i32 134217727>
 ; CHECK-NEXT:    ret <3 x i32> [[RET]]
 ;
   %tmp0 = shl <3 x i32> %x, <i32 5, i32 undef, i32 5>