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Teach basicaa that 'byval' arguments define a new memory location that

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can't be aliased to other known objects.  This allows us to know that byval 
pointer args don't alias globals, etc.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46315 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2008-01-24 18:00:32 +00:00
parent de6fa5f25f
commit fd68750038
2 changed files with 52 additions and 34 deletions
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68
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -138,9 +138,10 @@ ImmutablePass *llvm::createBasicAliasAnalysisPass() {
return new BasicAliasAnalysis();
}
// getUnderlyingObject - This traverses the use chain to figure out what object
// the specified value points to. If the value points to, or is derived from, a
// unique object or an argument, return it.
/// getUnderlyingObject - This traverses the use chain to figure out what object
/// the specified value points to. If the value points to, or is derived from,
/// a unique object or an argument, return it. This returns:
/// Arguments, GlobalVariables, Functions, Allocas, Mallocs.
static const Value *getUnderlyingObject(const Value *V) {
if (!isa<PointerType>(V->getType())) return 0;
@ -241,41 +242,29 @@ static bool AddressMightEscape(const Value *V) {
//
AliasAnalysis::ModRefResult
BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
if (!isa<Constant>(P))
if (const AllocationInst *AI =
dyn_cast_or_null<AllocationInst>(getUnderlyingObject(P))) {
if (!isa<Constant>(P)) {
const Value *Object = getUnderlyingObject(P);
// Allocations and byval arguments are "new" objects.
if (isa<AllocationInst>(Object) ||
(isa<Argument>(Object) && cast<Argument>(Object)->hasByValAttr())) {
// Okay, the pointer is to a stack allocated object. If we can prove that
// the pointer never "escapes", then we know the call cannot clobber it,
// because it simply can't get its address.
if (!AddressMightEscape(AI))
if (!AddressMightEscape(Object))
return NoModRef;
// If this is a tail call and P points to a stack location, we know that
// the tail call cannot access or modify the local stack.
if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
if (CI->isTailCall() && isa<AllocaInst>(AI))
if (CI->isTailCall() && !isa<MallocInst>(Object))
return NoModRef;
}
}
// The AliasAnalysis base class has some smarts, lets use them.
return AliasAnalysis::getModRefInfo(CS, P, Size);
}
static bool isNoAliasArgument(const Argument *Arg) {
const Function *Func = Arg->getParent();
const ParamAttrsList *Attr = Func->getParamAttrs();
if (Attr) {
unsigned Idx = 1;
for (Function::const_arg_iterator I = Func->arg_begin(),
E = Func->arg_end(); I != E; ++I, ++Idx) {
if (&(*I) == Arg &&
Attr->paramHasAttr(Idx, ParamAttr::NoAlias))
return true;
}
}
return false;
}
// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
// as array references. Note that this function is heavily tail recursive.
// Hopefully we have a smart C++ compiler. :)
@ -317,9 +306,12 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
// If they are two different objects, and one is a noalias argument
// then they do not alias.
if (O1 != O2 && isNoAliasArgument(O1Arg))
if (O1 != O2 && O1Arg->hasNoAliasAttr())
return NoAlias;
// Byval arguments can't alias globals or other arguments.
if (O1 != O2 && O1Arg->hasByValAttr()) return NoAlias;
// Otherwise, nothing is known...
}
@ -329,16 +321,18 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
// If they are two different objects, and one is a noalias argument
// then they do not alias.
if (O1 != O2 && isNoAliasArgument(O2Arg))
if (O1 != O2 && O2Arg->hasNoAliasAttr())
return NoAlias;
// Byval arguments can't alias globals or other arguments.
if (O1 != O2 && O2Arg->hasByValAttr()) return NoAlias;
// Otherwise, nothing is known...
} else if (O1 != O2) {
if (!isa<Argument>(O1))
// If they are two different objects, and neither is an argument,
// we know that we have no alias...
return NoAlias;
} else if (O1 != O2 && !isa<Argument>(O1)) {
// If they are two different objects, and neither is an argument,
// we know that we have no alias.
return NoAlias;
}
// If they are the same object, they we can look at the indexes. If they
@ -347,9 +341,15 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
// can't tell anything.
}
if (!isa<Argument>(O1) && isa<ConstantPointerNull>(V2))
return NoAlias; // Unique values don't alias null
// Unique values don't alias null, except non-byval arguments.
if (isa<ConstantPointerNull>(V2)) {
if (const Argument *O1Arg = dyn_cast<Argument>(O1)) {
if (O1Arg->hasByValAttr())
return NoAlias;
} else {
return NoAlias;
}
}
if (isa<GlobalVariable>(O1) ||
(isa<AllocationInst>(O1) &&

18
test/Analysis/BasicAA/byval.ll Normal file
View File

@ -0,0 +1,18 @@
; RUN: llvm-as < %s | opt -gvn | llvm-dis | grep {ret i32 1}
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
target triple = "i686-apple-darwin8"
%struct.x = type { i32, i32, i32, i32 }
@g = weak global i32 0 ; <i32*> [#uses=1]
define i32 @foo(%struct.x* byval %a) nounwind {
entry:
%tmp1 = tail call i32 (...)* @bar( %struct.x* %a ) nounwind ; <i32> [#uses=0]
%tmp2 = getelementptr %struct.x* %a, i32 0, i32 0 ; <i32*> [#uses=2]
store i32 1, i32* %tmp2, align 4
store i32 2, i32* @g, align 4
%tmp4 = load i32* %tmp2, align 4 ; <i32> [#uses=1]
ret i32 %tmp4
}
declare i32 @bar(...)