//===---------------------------------------------------------------------===//
-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
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
--- /dev/null
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | FileCheck %s
+
+; CHECK: define i32 @foo
+; %y = sub i32 0, %x
+; %z = sdiv i32 %y, 337
+; ret i32 %y
+define i32 @foo(i32 %x) {
+ %y = sub i32 0, %x
+ %z = sdiv i32 %y, 337
+ ret i32 %y
+}
+
+; CHECK: define i32 @bar
+; %y = sdiv i32 %x, -337
+; ret i32 %y
+define i32 @bar(i32 %x) {
+ %y = sub nsw i32 0, %x
+ %z = sdiv i32 %y, 337
+ ret i32 %y
+}