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Answer to Philip Reames comments
- add check for volatile (probably unneeded, but I agree that we should be conservative about it). - strengthen condition from isUnordered() to isSimple(), as I don't understand well enough Unordered semantics (and it also matches the comment better this way) to be confident in the previous behaviour (thanks for catching that one, I had missed the case Monotonic/Unordered). - separate a condition in two. - lengthen comment about aliasing and loads - add tests in GVN/atomic.ll llvm-svn: 215943
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@ -407,21 +407,33 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
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// Values depend on loads if the pointers are must aliased. This means that
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// a load depends on another must aliased load from the same value.
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// One exception is atomic loads: a value can depend on an atomic load that it
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// does not alias with when this atomic load indicates that another thread may
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// be accessing the location.
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if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
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// Atomic loads have complications involved.
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// A monotonic load is OK if the query inst is itself not atomic.
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// FIXME: This is overly conservative.
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if (!LI->isUnordered()) {
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if (!QueryInst || LI->getOrdering() != Monotonic)
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if (!QueryInst)
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return MemDepResult::getClobber(LI);
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if (LI->getOrdering() != Monotonic)
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return MemDepResult::getClobber(LI);
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if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
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if (!QueryLI->isUnordered())
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if (!QueryLI->isSimple())
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return MemDepResult::getClobber(LI);
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if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
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if (!QuerySI->isUnordered())
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if (!QuerySI->isSimple())
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return MemDepResult::getClobber(LI);
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}
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// FIXME: this is overly conservative.
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// While volatile access cannot be eliminated, they do not have to clobber
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// non-aliasing locations, as normal accesses can for example be reordered
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// with volatile accesses.
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if (LI->isVolatile())
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return MemDepResult::getClobber(LI);
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AliasAnalysis::Location LoadLoc = AA->getLocation(LI);
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// If we found a pointer, check if it could be the same as our pointer.
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@ -481,16 +493,25 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
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// A monotonic store is OK if the query inst is itself not atomic.
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// FIXME: This is overly conservative.
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if (!SI->isUnordered()) {
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if (!QueryInst || SI->getOrdering() != Monotonic)
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if (!QueryInst)
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return MemDepResult::getClobber(SI);
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if (SI->getOrdering() != Monotonic)
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return MemDepResult::getClobber(SI);
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if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
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if (!QueryLI->isUnordered())
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if (!QueryLI->isSimple())
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return MemDepResult::getClobber(SI);
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if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
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if (!QuerySI->isUnordered())
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if (!QuerySI->isSimple())
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return MemDepResult::getClobber(SI);
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}
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// FIXME: this is overly conservative.
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// While volatile access cannot be eliminated, they do not have to clobber
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// non-aliasing locations, as normal accesses can for example be reordered
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// with volatile accesses.
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if (SI->isVolatile())
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return MemDepResult::getClobber(SI);
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// If alias analysis can tell that this store is guaranteed to not modify
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// the query pointer, ignore it. Use getModRefInfo to handle cases where
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// the query pointer points to constant memory etc.
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@ -78,3 +78,29 @@ entry:
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%x3 = add i32 %x, %x2
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ret i32 %x3
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}
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; GVN across monotonic store (allowed)
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define i32 @test7() nounwind uwtable ssp {
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; CHECK: test7
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; CHECK: add i32 %x, %x
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entry:
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%x = load i32* @y
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store atomic i32 %x, i32* @x monotonic, align 4
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%y = load i32* @y
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%z = add i32 %x, %y
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ret i32 %z
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}
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; GVN of an unordered across monotonic load (not allowed)
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define i32 @test8() nounwind uwtable ssp {
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; CHECK: test8
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; CHECK: add i32 %x, %y
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entry:
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%x = load atomic i32* @y unordered, align 4
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%clobber = load atomic i32* @x monotonic, align 4
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%y = load atomic i32* @y monotonic, align 4
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%z = add i32 %x, %y
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ret i32 %z
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}
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