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Relax the constraint more in MemoryDependencyAnalysis.cpp

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Even loads/stores that have a stronger ordering than monotonic can be safe.
The rule is no release-acquire pair on the path from the QueryInst, assuming that
the QueryInst is not atomic itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216771 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Robin Morisset
2014-08-29 20:32:58 +00:00
parent 22f3cb0dc4
commit 66b380b6b6
3 changed files with 147 additions and 64 deletions
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46
lib/Analysis/MemoryDependenceAnalysis.cpp
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@@ -370,6 +370,36 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
int64_t MemLocOffset = 0;
unsigned Limit = BlockScanLimit;
bool isInvariantLoad = false;
// We must be careful with atomic accesses, as they may allow another thread
// to touch this location, cloberring it. We are conservative: if the
// QueryInst is not a simple (non-atomic) memory access, we automatically
// return getClobber.
// If it is simple, we know based on the results of
// "Compiler testing via a theory of sound optimisations in the C11/C++11
// memory model" in PLDI 2013, that a non-atomic location can only be
// clobbered between a pair of a release and an acquire action, with no
// access to the location in between.
// Here is an example for giving the general intuition behind this rule.
// In the following code:
// store x 0;
// release action; [1]
// acquire action; [4]
// %val = load x;
// It is unsafe to replace %val by 0 because another thread may be running:
// acquire action; [2]
// store x 42;
// release action; [3]
// with synchronization from 1 to 2 and from 3 to 4, resulting in %val
// being 42. A key property of this program however is that if either
// 1 or 4 were missing, there would be a race between the store of 42
// either the store of 0 or the load (making the whole progam racy).
// The paper mentionned above shows that the same property is respected
// by every program that can detect any optimisation of that kind: either
// it is racy (undefined) or there is a release followed by an acquire
// between the pair of accesses under consideration.
bool HasSeenAcquire = false;
if (isLoad && QueryInst) {
LoadInst *LI = dyn_cast<LoadInst>(QueryInst);
if (LI && LI->getMetadata(LLVMContext::MD_invariant_load) != nullptr)
@@ -412,19 +442,21 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
// be accessing the location.
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
// Atomic loads have complications involved.
// A monotonic load is OK if the query inst is itself not atomic.
// A Monotonic (or higher) load is OK if the query inst is itself not atomic.
// An Acquire (or higher) load sets the HasSeenAcquire flag, so that any
// release store will know to return getClobber.
// FIXME: This is overly conservative.
if (!LI->isUnordered()) {
if (!QueryInst)
return MemDepResult::getClobber(LI);
if (LI->getOrdering() != Monotonic)
return MemDepResult::getClobber(LI);
if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
if (!QueryLI->isSimple())
return MemDepResult::getClobber(LI);
if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
if (!QuerySI->isSimple())
return MemDepResult::getClobber(LI);
if (isAtLeastAcquire(LI->getOrdering()))
HasSeenAcquire = true;
}
// FIXME: this is overly conservative.
@@ -490,19 +522,21 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
// Atomic stores have complications involved.
// A monotonic store is OK if the query inst is itself not atomic.
// A Monotonic store is OK if the query inst is itself not atomic.
// A Release (or higher) store further requires that no acquire load
// has been seen.
// FIXME: This is overly conservative.
if (!SI->isUnordered()) {
if (!QueryInst)
return MemDepResult::getClobber(SI);
if (SI->getOrdering() != Monotonic)
return MemDepResult::getClobber(SI);
if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
if (!QueryLI->isSimple())
return MemDepResult::getClobber(SI);
if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
if (!QuerySI->isSimple())
return MemDepResult::getClobber(SI);
if (HasSeenAcquire && isAtLeastRelease(SI->getOrdering()))
return MemDepResult::getClobber(SI);
}
// FIXME: this is overly conservative.