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implement the first part of PR8882: when lowering an inbounds

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gep to explicit addressing, we know that none of the intermediate
computation overflows.

This could use review: it seems that the shifts certainly wouldn't
overflow, but could the intermediate adds overflow if there is a 
negative index?

Previously the testcase would instcombine to:

define i1 @test(i64 %i) {
  %p1.idx.mask = and i64 %i, 4611686018427387903
  %cmp = icmp eq i64 %p1.idx.mask, 1000
  ret i1 %cmp
}

now we get:

define i1 @test(i64 %i) {
  %cmp = icmp eq i64 %i, 1000
  ret i1 %cmp
}



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125271 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
2011-02-10 07:11:16 +00:00
parent 81baf14fdf
commit 6cdf2ea98e
3 changed files with 33 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/Transforms/InstCombine/InstCombineAddSub.cpp
+15 -8
View File
@@ -399,6 +399,10 @@ Value *InstCombiner::EmitGEPOffset(User *GEP) {
const Type *IntPtrTy = TD.getIntPtrType(GEP->getContext());
Value *Result = Constant::getNullValue(IntPtrTy);
// If the GEP is inbounds, we know that none of the addressing operations will
// overflow in an unsigned sense.
bool isInBounds = cast<GEPOperator>(GEP)->isInBounds();
// Build a mask for high order bits.
unsigned IntPtrWidth = TD.getPointerSizeInBits();
uint64_t PtrSizeMask = ~0ULL >> (64-IntPtrWidth);
@@ -414,31 +418,34 @@ Value *InstCombiner::EmitGEPOffset(User *GEP) {
if (const StructType *STy = dyn_cast<StructType>(*GTI)) {
Size = TD.getStructLayout(STy)->getElementOffset(OpC->getZExtValue());
Result = Builder->CreateAdd(Result,
ConstantInt::get(IntPtrTy, Size),
GEP->getName()+".offs");
if (Size)
Result = Builder->CreateAdd(Result, ConstantInt::get(IntPtrTy, Size),
GEP->getName()+".offs",
isInBounds /*NUW*/);
continue;
}
Constant *Scale = ConstantInt::get(IntPtrTy, Size);
Constant *OC =
ConstantExpr::getIntegerCast(OpC, IntPtrTy, true /*SExt*/);
Scale = ConstantExpr::getMul(OC, Scale);
Scale = ConstantExpr::getMul(OC, Scale, isInBounds/*NUW*/);
// Emit an add instruction.
Result = Builder->CreateAdd(Result, Scale, GEP->getName()+".offs");
Result = Builder->CreateAdd(Result, Scale, GEP->getName()+".offs",
isInBounds /*NUW*/);
continue;
}
// Convert to correct type.
if (Op->getType() != IntPtrTy)
Op = Builder->CreateIntCast(Op, IntPtrTy, true, Op->getName()+".c");
if (Size != 1) {
Constant *Scale = ConstantInt::get(IntPtrTy, Size);
// We'll let instcombine(mul) convert this to a shl if possible.
Op = Builder->CreateMul(Op, Scale, GEP->getName()+".idx");
Op = Builder->CreateMul(Op, ConstantInt::get(IntPtrTy, Size),
GEP->getName()+".idx", isInBounds /*NUW*/);
}
// Emit an add instruction.
Result = Builder->CreateAdd(Op, Result, GEP->getName()+".offs");
Result = Builder->CreateAdd(Op, Result, GEP->getName()+".offs",
isInBounds /*NUW*/);
}
return Result;
}