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Put a threshold on the number of users PointerMayBeCaptured 

examines; fall back to a conservative answer if there are
more. This works around some several compile time problems
resulting from BasicAliasAnalysis calling PointerMayBeCaptured.

The value has been chosen arbitrarily.

This fixes rdar://7438917 and may partially address PR5708.

llvm-svn: 90905
This commit is contained in:
Dan Gohman 2009-12-08 23:59:12 +00:00
parent e19122ff01
commit 2d27b191d9
1 changed files with 16 additions and 0 deletions
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16
llvm/lib/Analysis/CaptureTracking.cpp
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@ -25,6 +25,16 @@
#include "llvm/Support/CallSite.h"
using namespace llvm;
/// As its comment mentions, PointerMayBeCaptured can be expensive.
/// However, it's not easy for BasicAA to cache the result, because
/// it's an ImmutablePass. To work around this, bound queries at a
/// fixed number of uses.
///
/// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
/// a cache. Then we can move the code from BasicAliasAnalysis into
/// that path, and remove this threshold.
static int const Threshold = 20;
/// PointerMayBeCaptured - Return true if this pointer value may be captured
/// by the enclosing function (which is required to exist). This routine can
/// be expensive, so consider caching the results. The boolean ReturnCaptures
@ -37,12 +47,18 @@ bool llvm::PointerMayBeCaptured(const Value *V,
assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
SmallVector<Use*, 16> Worklist;
SmallSet<Use*, 16> Visited;
int Count = 0;
for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
UI != UE; ++UI) {
Use *U = &UI.getUse();
Visited.insert(U);
Worklist.push_back(U);
// If there are lots of uses, conservativelty say that the value
// is captured to avoid taking too much compile time.
if (Count++ >= Threshold)
return true;
}
while (!Worklist.empty()) {