[PATCH] D57970: [WinEH] Allocate unique stack slots for xmm CSRs in funclets

Fri Feb 15 16:02:52 PST 2019

rnk added a comment.

In D57970#1397522 <https://reviews.llvm.org/D57970#1397522>, @andrew.w.kaylor wrote:

> I haven't had a lot more time to spend on this, but I did do a bit of investigation. I still only have a vague idea of what would be required to implement hard-coded offsets, but it didn't look to me like there is existing infrastructure for this so all of the things that just kind of magically worked with this patch (like growing the stack allocation and writing the .seh_savexmm directive) would need to be explicitly updated, right?
>
> On the other hand, it looked like it might be relatively easy to update the code that writes the savexmm directive to describe the actual location of the stack slot I'm using in the current version of this patch. If I was reading the code correctly, it looks like it's already using the offset between the parent stack pointer and the funclet stack pointer to figure out how to build the instruction that actually stores the register to the stack. So we should be able to do a similar calculation to fill out the savexmm directive correctly. Does that seem worth exploring?

I don't think it's possible, because the XMM registers are stored relative to RBP, which points into the stack frame of the parent function. I believe the savexmm directive offsets are always relative to the RSP at the end of the prologue, even if a frame pointer is in use, and that points into the frame of the funclet, which is usually empty (we always use a frame size of 32 or something).

Anyway, I think you are right that this intermediate state would fix the issue for a lot of folks. Typically you don't need to restore XMM registers to unwind the stack (it's OK if their contents are wrong), so this is only a problem if someone recovers from an exception thrown out of a catch block.

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