[PATCH] D114362: [DebugInfo][4/N] Adjust x86 location-list tests for instruction referencing

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

Orlando accepted this revision.
Orlando added a comment.
This revision is now accepted and ready to land.

Ok, LGTM with the duplicated test and folded-in limitation remark. Thanks!



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Comment at: llvm/test/DebugInfo/COFF/register-variables.ll:48
 ; ASM:         #DEBUG_VALUE: f:p <- $esi
-; ASM:         #DEBUG_VALUE: c <- $eax
+; ASM:         #DEBUG_VALUE: c <- undef
 ; ASM:         movl    %eax, %ecx
----------------
jmorse wrote:
> Orlando wrote:
> > Non-blocking question: why do we get `c <- undef` here? The non-instr-ref location `c <- $eax` looks correct AFAICT.
> Looking at the IR, there's some tail-folding happening, where two instances of:
> 
>   dbg.value
>   tail call void @putint(
> 
> on either side of a branch get merged together. In DBG_VALUE mode, I believe this works by coincidence: the tails get merged, debug instructions ignored, and the variable location preserved because one code sequence is spliced into the common tail. If there was only a dbg.value on one side, we'd end up with a false location.
> 
> With instruction referencing there's still one DBG_INSTR_REF preserved, but LiveDebugValues can tell that it isn't dominated by the definition it's referring to, so drops the location.
> 
> IMO this is a good demonstration that instruction referencing soundly drops questionable locations; exactly how to fix it is a wider question, and probably needs a little more instrumentation. I'll fold a note in remarking on the limitation.
Aha, that is quite interesting. Thanks for the explanation.


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