SmallPtrSetImpl<Function *> *Writers = nullptr,
GlobalValue *OkayStoreDest = nullptr);
bool AnalyzeIndirectGlobalMemory(GlobalValue *GV);
+
+ bool isNonEscapingGlobalNoAlias(const GlobalValue *GV, const Value *V);
};
}
}
}
+// There are particular cases where we can conclude no-alias between
+// a non-addr-taken global and some other underlying object. Specifically,
+// a non-addr-taken global is known to not be escaped from any function. It is
+// also incorrect for a transformation to introduce an escape of a global in
+// a way that is observable when it was not there previously. One function
+// being transformed to introduce an escape which could possibly be observed
+// (via loading from a global or the return value for example) within another
+// function is never safe. If the observation is made through non-atomic
+// operations on different threads, it is a data-race and UB. If the
+// observation is well defined, by being observed the transformation would have
+// changed program behavior by introducing the observed escape, making it an
+// invalid transform.
+//
+// This property does require that transformations which *temporarily* escape
+// a global that was not previously escaped, prior to restoring it, cannot rely
+// on the results of GMR::alias. This seems a reasonable restriction, although
+// currently there is no way to enforce it. There is also no realistic
+// optimization pass that would make this mistake. The closest example is
+// a transformation pass which does reg2mem of SSA values but stores them into
+// global variables temporarily before restoring the global variable's value.
+// This could be useful to expose "benign" races for example. However, it seems
+// reasonable to require that a pass which introduces escapes of global
+// variables in this way to either not trust AA results while the escape is
+// active, or to be forced to operate as a module pass that cannot co-exist
+// with an alias analysis such as GMR.
+bool GlobalsModRef::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
+ const Value *V) {
+ // In order to know that the underlying object cannot alias the
+ // non-addr-taken global, we must know that it would have to be an escape.
+ // Thus if the underlying object is a function argument, a load from
+ // a global, or the return of a function, it cannot alias. We can also
+ // recurse through PHI nodes and select nodes provided all of their inputs
+ // resolve to one of these known-escaping roots.
+ SmallPtrSet<const Value *, 8> Visited;
+ SmallVector<const Value *, 8> Inputs;
+ Visited.insert(V);
+ Inputs.push_back(V);
+ int Depth = 0;
+ do {
+ const Value *Input = Inputs.pop_back_val();
+
+ if (auto *InputGV = dyn_cast<GlobalValue>(Input)) {
+ // If one input is the very global we're querying against, then we can't
+ // conclude no-alias.
+ if (InputGV == GV)
+ return false;
+
+ // FIXME: It would be good to handle other obvious no-alias cases here, but
+ // it isn't clear how to do so reasonbly without building a small version
+ // of BasicAA into this code. We could recurse into AliasAnalysis::alias
+ // here but that seems likely to go poorly as we're inside the
+ // implementation of such a query. Until then, just conservatievly retun
+ // false.
+ return false;
+ }
+
+ if (isa<Argument>(Input) || isa<CallInst>(Input) ||
+ isa<InvokeInst>(Input)) {
+ // Arguments to functions or returns from functions are inherently
+ // escaping, so we can immediately classify those as not aliasing any
+ // non-addr-taken globals.
+ continue;
+ }
+ if (auto *LI = dyn_cast<LoadInst>(Input)) {
+ // A pointer loaded from a global would have been captured, and we know
+ // that the global is non-escaping, so no alias.
+ if (isa<GlobalValue>(LI->getPointerOperand()))
+ continue;
+
+ // Otherwise, a load could come from anywhere, so bail.
+ return false;
+ }
+
+ // Recurse through a limited number of selects and PHIs. This is an
+ // arbitrary depth of 4, lower numbers could be used to fix compile time
+ // issues if needed, but this is generally expected to be only be important
+ // for small depths.
+ if (++Depth > 4)
+ return false;
+ if (auto *SI = dyn_cast<SelectInst>(Input)) {
+ const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), *DL);
+ const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), *DL);
+ if (Visited.insert(LHS).second)
+ Inputs.push_back(LHS);
+ if (Visited.insert(RHS).second)
+ Inputs.push_back(RHS);
+ continue;
+ }
+ if (auto *PN = dyn_cast<PHINode>(Input)) {
+ for (const Value *Op : PN->incoming_values()) {
+ Op = GetUnderlyingObject(Op, *DL);
+ if (Visited.insert(Op).second)
+ Inputs.push_back(Op);
+ }
+ continue;
+ }
+
+ // Unknown instruction, bail.
+ return false;
+ } while (!Inputs.empty());
+
+ // If all the inputs to V were definitively no-alias, then V is no-alias.
+ return true;
+}
+
/// alias - If one of the pointers is to a global that we are tracking, and the
/// other is some random pointer, we know there cannot be an alias, because the
/// address of the global isn't taken.
if ((GV1 || GV2) && GV1 != GV2)
return NoAlias;
- // There are particular cases where we can conclude no-alias between
- // a non-addr-taken global and some other underlying object. Specifically,
- // a non-addr-taken global is known to not be escaped from any function. It
- // is also incorrect for a transformation to introduce an escape of
- // a global in a way that is observable when it was not there previously.
- // One function being transformed to introduce an escape which could
- // possibly be observed (via loading from a global or the return value for
- // example) within another function is never safe. If the observation is
- // made through non-atomic operations on different threads, it is
- // a data-race and UB. If the observation is well defined, by being
- // observed the transformation would have changed program behavior by
- // introducing the observed escape, making it an invalid transform.
- //
- // This property does require that transformations which *temporarily*
- // escape a global that was not previously escaped, prior to restoring
- // it, cannot rely on the results of GMR::alias. This seems a reasonable
- // restriction, although currently there is no way to enforce it. There is
- // also no realistic optimization pass that would make this mistake. The
- // closest example is a transformation pass which does reg2mem of SSA
- // values but stores them into global variables temporarily before
- // restoring the global variable's value. This could be useful to expose
- // "benign" races for example. However, it seems reasonable to require that
- // a pass which introduces escapes of global variables in this way to
- // either not trust AA results while the escape is active, or to be forced
- // to operate as a module pass that cannot co-exist with an alias analysis
- // such as GMR.
+ // Check for a special case where a non-escaping global can be used to
+ // conclude no-alias.
if ((GV1 || GV2) && GV1 != GV2) {
+ const GlobalValue *GV = GV1 ? GV1 : GV2;
const Value *UV = GV1 ? UV2 : UV1;
-
- // In order to know that the underlying object cannot alias the
- // non-addr-taken global, we must know that it would have to be an
- // escape. Thus if the underlying object is a function argument, a load
- // from a global, or the return of a function, it cannot alias.
- if (isa<Argument>(UV) || isa<CallInst>(UV) || isa<InvokeInst>(UV)) {
- // Arguments to functions or returns from functions are inherently
- // escaping, so we can immediately classify those as not aliasing any
- // non-addr-taken globals.
+ if (isNonEscapingGlobalNoAlias(GV, UV))
return NoAlias;
- } else if (auto *LI = dyn_cast<LoadInst>(UV)) {
- // A pointer loaded from a global would have been captured, and we know
- // that GV is non-addr-taken, so no alias.
- if (isa<GlobalValue>(LI->getPointerOperand()))
- return NoAlias;
- }
}
// Otherwise if they are both derived from the same addr-taken global, we
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