[PATCH] D11815: Pass subtarget feature "force-align-stack"

[Vasileios Kalintiris via cfe-commits]

vkalintiris added a comment.

In http://reviews.llvm.org/D11815#237541, @ahatanak wrote:

> In http://reviews.llvm.org/D11815#236368, @vkalintiris wrote:
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> > In http://reviews.llvm.org/D11815#235394, @ahatanak wrote:
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> > For example, on a Mips target, where the O32 ABI requires either way an 8-byte alignment, we would generate redundant code for realigning the stack to a 4-byte alignment if a function contains objects with maximum alignment of 4-bytes (see attached files to get an idea).
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> I wonder if there is a target or a use case that requires or prefers realigning the stack to an alignment that is smaller than the default stack alignment.  If there is no such target or use case, I think we can just using the existing attribute StackAlignment (with value 0) rather than adding a new function attribute "stackrealign", which will ensure the stack is at least aligned to the default value and force realigning the stack.


I was thinking something similar. I'm not quite sure but the only case I can think of, where we might get away with an alignment smaller than the default, is when we emit code for the fast calling convention. Other than that, I believe that we should duplicate the functionality of X86FrameLowering::calculateMaxStackAlign() (or override it in each target accordingly).

> The purpose of this patch is to make sure -mstackrealign works when doing LTO


Out of curiosity, I was wondering why dynamic stack realignment isn't enough for LTO. I would guess that the alignment of the data types used under SSE might have a smaller alignment than the one required by the ABI.


http://reviews.llvm.org/D11815