[PATCH] D90046: [LiveDebugValues] Handle spill locations with a fixed and scalable component.

[Sander de Smalen via Phabricator via llvm-commits]

sdesmalen added a comment.

> Are there any scenarios where stack slots can be accessed with different scalable offsets? For example if stack slot colouring merged two slots for vectors of different scalable size (maybe it doesn't do that). If that happened, the StackOffset object would compare differently, and we might miss a spilt value being overwritten (see llvm/test/DebugInfo/MIR/X86/live-debug-values-stack-clobber.mir for a scalar example of overwrites).

I guess that is possible. As long as both frame objects have the same Stack-ID, StackSlotColouring can merge them together. However, in that case, the sizes and offsets of those objects would both be scalable and comparing these sizes would be no different as they would be for non-scalable sizes.

> The size of a VarLoc object increases from 72 bytes to ~80 bytes on amd64; this isn't ideal, but trying to micro-optimise it at this stage invites errors.

Yes, that is unfortunate. We'll need to accept some cost for returning structs instead of `int/unsigned` throughout the code-base to represent the scalable semantics (same for TypeSize and ElementCount).



================
Comment at: llvm/test/CodeGen/AArch64/live-debugvalues-sve.mir:148-150
+    ADJCALLSTACKUP 0, 0, implicit-def dead $sp, implicit $sp, debug-location !34
+    renamable $z2 = COPY $z0, debug-location !34
+    B %bb.3, debug-location !35
----------------
jmorse wrote:
> I'd recommend also testing restoring the vector to a register, to ensure LiveDebugValues chops the spill-expression off correctly. VarLocBasedLDV should put in a DBG_VALUE after loading from the stack; you should need to clobber the stack location with some other value to force InstrRefBasedLDV to issue another DBG_VALUE.
I tried this, but I'm not entirely sure the result is correct because `isRestoreInstruction` always returns false (`TargetInstrInfo::getRestoreSize` always returns `0` if this is not overriden by the target, which it is not for AArch64).

So when I do a reload from %stack.0 and then clobber that stack location, like this:
```$z3 = IMPLICIT_DEF
renamable $z1 = LD1W_IMM renamable $p0, %stack.0, 0, debug-location !34 :: (load unknown-size from %stack.0, align 16)
ST1W_IMM renamable $z3, killed renamable $p0, %stack.0, 0 :: (store unknown-size into %stack.0, align 16)
```

LiveDebugValues creates `DBG_VALUE $noreg, $noreg, !27, !DIExpression(), debug-location !30`. It doesn't seem to recognize LD1W_IMM as a reload, and therefore doesn't see that we've just reloaded debug-value `!27` back into `$z1`. The DIExpression is empty though.

Is that what you meant?


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