[PATCH] D47980: [InstCombine] Fold (x << y) >> y -> x & (-1 >> y)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, craig.topper.

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


Repository:
  rL LLVM

https://reviews.llvm.org/D47980

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


Index: test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll
===================================================================
--- test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll
+++ test/Transforms/InstCombine/canonicalize-shl-lshr-to-masking.ll
@@ -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
@@ -113,8 +113,8 @@
 
 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
@@ -160,8 +160,7 @@
 
 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>
Index: lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -802,6 +802,16 @@
       return &I;
     }
   }
+
+  // Transform  (x << y) >> y  to  x & (-1 >> y)
+  Value *X, *Y;
+  if (match(&I,
+            m_LShr(m_OneUse(m_Shl(m_Value(X), m_Value(Y))), m_Deferred(Y)))) {
+    Constant *AllOnes = ConstantInt::getAllOnesValue(Ty);
+    Value *Mask = Builder.CreateLShr(AllOnes, Y);
+    return BinaryOperator::CreateAnd(Mask, X);
+  }
+
   return nullptr;
 }
 


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