Remove the hack that turns sdiv by a power of 2 to ashr, and
use the new "exact" sdiv to allow LLVM optimization to perform
this transformation.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78739 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGExprScalar.cpp b/lib/CodeGen/CGExprScalar.cpp
index e761e9f..f8fae42 100644
--- a/lib/CodeGen/CGExprScalar.cpp
+++ b/lib/CodeGen/CGExprScalar.cpp
@@ -1184,19 +1184,12 @@
// Optimize out the shift for element size of 1.
if (ElementSize == 1)
return BytesBetween;
-
- // HACK: LLVM doesn't have an divide instruction that 'knows' there is no
- // remainder. As such, we handle common power-of-two cases here to generate
- // better code. See PR2247.
- if (llvm::isPowerOf2_64(ElementSize)) {
- Value *ShAmt =
- llvm::ConstantInt::get(ResultType, llvm::Log2_64(ElementSize));
- return Builder.CreateAShr(BytesBetween, ShAmt, "sub.ptr.shr");
- }
-
- // Otherwise, do a full sdiv.
+
+ // Otherwise, do a full sdiv. This uses the "exact" form of sdiv, since
+ // pointer difference in C is only defined in the case where both
+ // operands are pointing to elements of an array.
Value *BytesPerElt = llvm::ConstantInt::get(ResultType, ElementSize);
- return Builder.CreateSDiv(BytesBetween, BytesPerElt, "sub.ptr.div");
+ return Builder.CreateExactSDiv(BytesBetween, BytesPerElt, "sub.ptr.div");
}
}