From: Sanjay Patel <spatel@rotateright.com>
Date: Fri, 21 Apr 2017 14:03:54 +0000 (+0000)
Subject: [InstCombine] prefer xor with -1 because 'not' is easier to understand (PR32706)
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=bd1b03d467c2a5f96ccf612d9ae1c31a02a654ea;p=llvm

[InstCombine] prefer xor with -1 because 'not' is easier to understand (PR32706)

This matches the demanded bits behavior in the DAG and should fix:
https://bugs.llvm.org/show_bug.cgi?id=32706

Differential Revision: https://reviews.llvm.org/D32255


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300977 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 2f6e411345f..d7e709c9cca 100644
--- a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -277,10 +277,20 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
       return InsertNewInstWith(And, *I);
     }
 
-    // If the RHS is a constant, see if we can simplify it.
-    // FIXME: for XOR, we prefer to force bits to 1 if they will make a -1.
-    if (ShrinkDemandedConstant(I, 1, DemandedMask))
-      return I;
+    // If the RHS is a constant, see if we can change it. Don't alter a -1
+    // constant because that's a canonical 'not' op, and that is better for
+    // combining, SCEV, and codegen.
+    const APInt *C;
+    if (match(I->getOperand(1), m_APInt(C)) && !C->isAllOnesValue()) {
+      if ((*C | ~DemandedMask).isAllOnesValue()) {
+        // Force bits to 1 to create a 'not' op.
+        I->setOperand(1, ConstantInt::getAllOnesValue(VTy));
+        return I;
+      }
+      // If we can't turn this into a 'not', try to shrink the constant.
+      if (ShrinkDemandedConstant(I, 1, DemandedMask))
+        return I;
+    }
 
     // If our LHS is an 'and' and if it has one use, and if any of the bits we
     // are flipping are known to be set, then the xor is just resetting those
diff --git a/test/Transforms/InstCombine/xor.ll b/test/Transforms/InstCombine/xor.ll
index 570155b1623..33fe557ae5a 100644
--- a/test/Transforms/InstCombine/xor.ll
+++ b/test/Transforms/InstCombine/xor.ll
@@ -536,3 +536,20 @@ define i32 @test38(i32 %A, i32 %B) {
   %xor = xor i32 %and, %B
   ret i32 %xor
 }
+
+; PR32706 - https://bugs.llvm.org/show_bug.cgi?id=32706
+; Pin an xor constant operand to -1 if possible because 'not' is better for SCEV and codegen.
+
+define i32 @not_is_canonical(i32 %x, i32 %y) {
+; CHECK-LABEL: @not_is_canonical(
+; CHECK-NEXT:    [[SUB:%.*]] = xor i32 %x, -1
+; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[SUB]], %y
+; CHECK-NEXT:    [[MUL:%.*]] = shl i32 [[ADD]], 2
+; CHECK-NEXT:    ret i32 [[MUL]]
+;
+  %sub = xor i32 %x, 1073741823
+  %add = add i32 %sub, %y
+  %mul = shl i32 %add, 2
+  ret i32 %mul
+}
+
diff --git a/test/Transforms/InstCombine/zext.ll b/test/Transforms/InstCombine/zext.ll
index 887d839cb8c..d5c52bd811b 100644
--- a/test/Transforms/InstCombine/zext.ll
+++ b/test/Transforms/InstCombine/zext.ll
@@ -35,7 +35,7 @@ define <2 x i64> @test3(<2 x i64> %A) {
 
 define <2 x i64> @test4(<2 x i64> %A) {
 ; CHECK-LABEL: @test4(
-; CHECK-NEXT:    [[TMP1:%.*]] = xor <2 x i64> %A, <i64 63, i64 63>
+; CHECK-NEXT:    [[TMP1:%.*]] = xor <2 x i64> %A, <i64 -1, i64 -1>
 ; CHECK-NEXT:    [[XOR:%.*]] = and <2 x i64> [[TMP1]], <i64 23, i64 42>
 ; CHECK-NEXT:    ret <2 x i64> [[XOR]]
 ;