[LLVMdev] ValueTracking's GetUnderlyingObject vs. ScheduleDAGInstrs' getUnderlyingObject

Arnold Schwaighofer arnolds at codeaurora.org
Mon Oct 15 09:26:03 PDT 2012

It seems to me that this is a bug in StackColoring. It should not
assume that GetUnderlyingObject always returns the underlying alloca
(it might fail for several reason - e.g. the default max lookup depth
is 6).

// Climb up and find the original alloca.
00514         V = GetUnderlyingObject(V);
00515         // If we did not find one, or if the one that we found
is not in our
00516         // map, then move on.
00517         if (!V || !Allocas.count(V))
00518           continue;
00520         MMO->setValue(Allocas[V]);
00521         FixedMemOp++;

Instead of continuing, it should probably clean the Memory Operand so
that future alias queries will return a conservative result.

On Mon, Oct 15, 2012 at 11:00 AM, Matthew Curtis <mcurtis at codeaurora.org> wrote:
> Hello all.
> I'm investigating a problem with "Machine Instruction Scheduling" that is
> rooted in incorrect alias information.
> Things go wrong in the "Merge disjoint stack slots"[1] pass when a store
> instruction fails to get updated after its stack slot is merged.  As a
> result, when "Machine Instruction Scheduling" runs, it incorrectly re-orders
> the store and a subsequent load, thinking that they do not refer to the same
> underlying object when they actually do.
> The logic in "Merge disjoint stack slots" seems ok, except that it relies on
> llvm::GetUnderlyingObject[2] to determine if an instruction needs to be
> updated. Unfortunately, unlike ScheduleDAGInstrs' getUnderlyingObject[3],
> llvm::GetUnderlyingObject doesn't handle ptrtoint instructions, and in this
> case, fails to see that the problematic store refers to the merged stack
> slot.
> It seems to me that the logic in ScheduleDAGInstrs's getUnderlyingObject
> should be pushed into llvm::GetUnderlyingObject. And indeed, when I do this,
> "Merge disjoint stack slots" correctly updates all of the instructions and
> "Machine Instruction Scheduling" behaves correctly.
> Is this the right thing to do? Would other callers to
> llvm::GetUnderlyingObject not want the additional behavior?
> Thanks,
> Matthew Curtis.
> [1] http://llvm.org/docs/doxygen/html/StackColoring_8cpp_source.html
> [2] http://llvm.org/docs/doxygen/html/ValueTracking_8cpp_source.html#l01780
> [3]
> http://llvm.org/docs/doxygen/html/ScheduleDAGInstrs_8cpp_source.html#l00087
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