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Add a hack (mirroring llvm-gcc) to pointer difference

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codegen to compile:

int test(int *A, int *B) {
  return A-B;
}

into:

_test:
        movl 4(%esp), %eax
        subl 8(%esp), %eax
        sarl $2, %eax
        ret

instead of:

_test:
        movl 4(%esp), %eax
        subl 8(%esp), %eax
        movl %eax, %ecx
        sarl $31, %ecx
        shrl $30, %ecx
        addl %ecx, %eax
        sarl $2, %eax
        ret

llvm-svn: 39902
This commit is contained in:
Chris Lattner 2007-07-16 05:43:05 +00:00
parent f57999dcb1
commit 651f0e9c8f
1 changed files with 17 additions and 7 deletions
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24
clang/CodeGen/CGExpr.cpp
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@ -18,6 +18,7 @@
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Support/MathExtras.h"
using namespace clang;
using namespace CodeGen;
@ -1168,7 +1169,7 @@ RValue CodeGenFunction::EmitPointerSub(RValue LHS, QualType LHSTy,
QualType LHSElementType = LHSPtrType->getPointeeType();
assert(LHSElementType == RHSPtrType->getPointeeType() &&
"can't subtract pointers with differing element types");
unsigned ElementSize = getContext().getTypeSize(LHSElementType,
uint64_t ElementSize = getContext().getTypeSize(LHSElementType,
SourceLocation()) / 8;
const llvm::Type *ResultType = ConvertType(ResTy);
llvm::Value *CastLHS = Builder.CreatePtrToInt(LHSValue, ResultType,
@ -1177,12 +1178,21 @@ RValue CodeGenFunction::EmitPointerSub(RValue LHS, QualType LHSTy,
"sub.ptr.rhs.cast");
llvm::Value *BytesBetween = Builder.CreateSub(CastLHS, CastRHS,
"sub.ptr.sub");
llvm::Value *BytesPerElement = llvm::ConstantInt::get(ResultType,
ElementSize);
llvm::Value *ElementsBetween = Builder.CreateSDiv(BytesBetween,
BytesPerElement,
"sub.ptr.div");
return RValue::get(ElementsBetween);
// 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.
if (llvm::isPowerOf2_64(ElementSize)) {
llvm::Value *ShAmt =
llvm::ConstantInt::get(ResultType, llvm::Log2_64(ElementSize));
return RValue::get(Builder.CreateAShr(BytesBetween, ShAmt,"sub.ptr.shr"));
} else {
// Otherwise, do a full sdiv.
llvm::Value *BytesPerElement =
llvm::ConstantInt::get(ResultType, ElementSize);
return RValue::get(Builder.CreateSDiv(BytesBetween, BytesPerElement,
"sub.ptr.div"));
}
} else {
// pointer - int
llvm::Value *NegatedRHS = Builder.CreateNeg(RHSValue, "sub.ptr.neg");