[PATCH] D94542: [X86] Default to -x86-pad-for-align=false to drop assembler difference with or w/o -g

Thu Jan 14 21:39:32 PST 2021

reames added a comment.

In D94542#2500116 <https://reviews.llvm.org/D94542#2500116>, @dblaikie wrote:

> In D94542#2500058 <https://reviews.llvm.org/D94542#2500058>, @reames wrote:
>
>> 
>
> ... is pretty important that turning on debug info does not change the generated machine code - otherwise there's the chance of heisenbugs.
>
> If this is a case where enabling debug info changes the selected machine code, that is a bug and one worth fixing one way or another.

David, thanks for chiming in here.  Reading your wording made me realize I was wrong in my take here.  I had been thinking of this as a debug info quality problem, but you're right, that's not what is actually going on here.

> If those backing the optimization aren't willing to sign up to do that work, then I think it's plausible to suggest that the optimization isn't feasible to add at this time.

I don't have the bandwidth to take this on.

This does leave us in an unfortunate place where to mitigate the JCC errata performance issues you need to enable a flag which introduces the heisenbugs David mentions, but I really don't see a way around that.  That's not an argument to leave all users effected by the issue.

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https://reviews.llvm.org/D94542