[PATCH] Fix AArch64 prologue for empty frame with dynamic allocas

[Tim Northover]

I think there's more going on here that needs fixing too. For example if I add a simple "%tmp = alloca i8, i32 4" to your entry block I get the same assertion failure David mentioned in the PR (even with your patch).

To me it looks like NeedsRealignment is misdefined, though that might not be the end of it. Elsewhere we used RegInfo->needsStackRealignment(MF), which looks like it would be an improvement, but doesn't actually fix the issue.

Another possible problem in this area is that if NumBytes != 0, we end up realigning the stack twice: once in the prologue and once at the alloca. That's arguably a separate performance problem though.

Cheers.

Tim.


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http://reviews.llvm.org/D10826

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