[PATCH] D31120: [InstCombine] Teach SimplifyDemandedUseBits that adding or subtractings 0s from every bit below the highest demanded bit can be simplified

[Craig Topper via Phabricator via llvm-commits]

craig.topper updated this revision to Diff 93705.
craig.topper added a comment.

Found another way of testing this particular transform.


https://reviews.llvm.org/D31120

Files:
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
  test/Transforms/InstCombine/and2.ll


Index: test/Transforms/InstCombine/and2.ll
===================================================================
--- test/Transforms/InstCombine/and2.ll
+++ test/Transforms/InstCombine/and2.ll
@@ -126,8 +126,7 @@
 define i32 @test11(i32 %a, i32 %b) {
 ; CHECK-LABEL: @test11(
 ; CHECK-NEXT:    [[X:%.*]] = shl i32 [[A:%.*]], 8
-; CHECK-NEXT:    [[Y:%.*]] = add i32 [[X]], [[B:%.*]]
-; CHECK-NEXT:    [[Z:%.*]] = and i32 [[Y]], 128
+; CHECK-NEXT:    [[Z:%.*]] = and i32 [[B:%.*]], 128
 ; CHECK-NEXT:    [[W:%.*]] = mul i32 [[Z]], [[X]]
 ; CHECK-NEXT:    ret i32 [[W]]
 ;
@@ -142,8 +141,7 @@
 define i32 @test12(i32 %a, i32 %b) {
 ; CHECK-LABEL: @test12(
 ; CHECK-NEXT:    [[X:%.*]] = shl i32 [[A:%.*]], 8
-; CHECK-NEXT:    [[Y:%.*]] = add i32 [[X]], [[B:%.*]]
-; CHECK-NEXT:    [[Z:%.*]] = and i32 [[Y]], 128
+; CHECK-NEXT:    [[Z:%.*]] = and i32 [[B:%.*]], 128
 ; CHECK-NEXT:    [[W:%.*]] = mul i32 [[Z]], [[X]]
 ; CHECK-NEXT:    ret i32 [[W]]
 ;
@@ -158,8 +156,7 @@
 define i32 @test13(i32 %a, i32 %b) {
 ; CHECK-LABEL: @test13(
 ; CHECK-NEXT:    [[X:%.*]] = shl i32 [[A:%.*]], 8
-; CHECK-NEXT:    [[Y:%.*]] = sub i32 [[B:%.*]], [[X]]
-; CHECK-NEXT:    [[Z:%.*]] = and i32 [[Y]], 128
+; CHECK-NEXT:    [[Z:%.*]] = and i32 [[B:%.*]], 128
 ; CHECK-NEXT:    [[W:%.*]] = mul i32 [[Z]], [[X]]
 ; CHECK-NEXT:    ret i32 [[W]]
 ;
Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -538,7 +538,7 @@
           SimplifyDemandedBits(I, 0, DemandedFromOps, LHSKnownZero, LHSKnownOne,
                                Depth + 1) ||
           ShrinkDemandedConstant(I, 1, DemandedFromOps) ||
-          SimplifyDemandedBits(I, 1, DemandedFromOps, LHSKnownZero, LHSKnownOne,
+          SimplifyDemandedBits(I, 1, DemandedFromOps, RHSKnownZero, RHSKnownOne,
                                Depth + 1)) {
         // Disable the nsw and nuw flags here: We can no longer guarantee that
         // we won't wrap after simplification. Removing the nsw/nuw flags is
@@ -548,6 +548,15 @@
         BinOP.setHasNoUnsignedWrap(false);
         return I;
       }
+
+      // If we are known to be adding/subtracting zeros to every bit below
+      // the highest demanded bit, we just return the other side.
+      if ((DemandedFromOps & RHSKnownZero) == DemandedFromOps)
+        return I->getOperand(0);
+      // We can't do this with the LHS for subtraction.
+      if (I->getOpcode() == Instruction::Add &&
+          (DemandedFromOps & LHSKnownZero) == DemandedFromOps)
+        return I->getOperand(1);
     }
 
     // Otherwise just hand the add/sub off to computeKnownBits to fill in


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