Transform -X/C to X/-C, implementing a README.txt entry.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78812 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 559835e..97546c4 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1111,16 +1111,6 @@
//===---------------------------------------------------------------------===//
-We would like to do the following transform in the instcombiner:
-
- -X/C -> X/-C
-
-However, this isn't valid if (-X) overflows. We can implement this when we
-have the concept of a "C signed subtraction" operator that which is undefined
-on overflow.
-
-//===---------------------------------------------------------------------===//
-
This was noticed in the entryblock for grokdeclarator in 403.gcc:
%tmp = icmp eq i32 %decl_context, 4
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 669ef2d..b875420 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -3077,6 +3077,14 @@
RHS->getValue().exactLogBase2());
return BinaryOperator::CreateAShr(Op0, ShAmt, I.getName());
}
+
+ // -X/C --> X/-C provided the negation doesn't overflow.
+ if (SubOperator *Sub = dyn_cast<SubOperator>(Op0))
+ if (isa<Constant>(Sub->getOperand(0)) &&
+ cast<Constant>(Sub->getOperand(0))->isNullValue() &&
+ Sub->hasNoSignedOverflow())
+ return BinaryOperator::CreateSDiv(Sub->getOperand(1),
+ ConstantExpr::getNeg(RHS));
}
// If the sign bits of both operands are zero (i.e. we can prove they are