[PATCH] D47981: [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 matching splat constants, but not just values.

Further improvements for non-matching splat constants, as noted in
https://reviews.llvm.org/D46760#1123713 will be needed,
but i'd prefer to do that as a follow-up.

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


Repository:
  rL LLVM

https://reviews.llvm.org/D47981

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


Index: test/Transforms/InstCombine/canonicalize-lshr-shl-to-masking.ll
===================================================================
--- test/Transforms/InstCombine/canonicalize-lshr-shl-to-masking.ll
+++ test/Transforms/InstCombine/canonicalize-lshr-shl-to-masking.ll
@@ -15,8 +15,8 @@
 
 define i32 @positive_samevar(i32 %x, i32 %y) {
 ; CHECK-LABEL: @positive_samevar(
-; CHECK-NEXT:    [[TMP0:%.*]] = lshr i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[RET:%.*]] = shl i32 [[TMP0]], [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = shl i32 -1, [[Y:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = and i32 [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret i32 [[RET]]
 ;
   %tmp0 = lshr i32 %x, %y
@@ -114,8 +114,8 @@
 
 define <2 x i32> @positive_samevar_vec(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @positive_samevar_vec(
-; CHECK-NEXT:    [[TMP0:%.*]] = lshr <2 x i32> [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[RET:%.*]] = shl <2 x i32> [[TMP0]], [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <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 = lshr <2 x i32> %x, %y
@@ -161,8 +161,7 @@
 
 define <3 x i32> @positive_sameconst_vec_undef2(<3 x i32> %x) {
 ; CHECK-LABEL: @positive_sameconst_vec_undef2(
-; CHECK-NEXT:    [[TMP0:%.*]] = lshr <3 x i32> [[X:%.*]], <i32 5, i32 undef, i32 5>
-; CHECK-NEXT:    [[RET:%.*]] = shl <3 x i32> [[TMP0]], <i32 5, i32 undef, i32 5>
+; CHECK-NEXT:    [[RET:%.*]] = and <3 x i32> [[X:%.*]], <i32 -32, i32 undef, i32 -32>
 ; CHECK-NEXT:    ret <3 x i32> [[RET]]
 ;
   %tmp0 = lshr <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
@@ -600,6 +600,7 @@
   if (Instruction *V = commonShiftTransforms(I))
     return V;
 
+  Type *Ty = I.getType();
   const APInt *ShAmtAPInt;
   if (match(Op1, m_APInt(ShAmtAPInt))) {
     unsigned ShAmt = ShAmtAPInt->getZExtValue();
@@ -670,6 +671,15 @@
     }
   }
 
+  // Transform  (x >> y) << y  to  x & (-1 << y)
+  Value *X, *Y;
+  if (match(&I,
+            m_Shl(m_OneUse(m_LShr(m_Value(X), m_Value(Y))), m_Deferred(Y)))) {
+    Constant *AllOnes = ConstantInt::getAllOnesValue(Ty);
+    Value *Mask = Builder.CreateShl(AllOnes, Y);
+    return BinaryOperator::CreateAnd(Mask, X);
+  }
+
   Constant *C1;
   if (match(Op1, m_Constant(C1))) {
     Constant *C2;


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