[LLVMdev] Question about callee saved registers in x86

[Pasi Parviainen]

On 28.5.2014 2:57, Sanjoy Das wrote:
> Hi llvmdev,
>
> I'm trying to figure how llvm remembers stack slots allotted to callee
> saved registers on x86.  In particular, llvm pushes registers in
> decreasing order of FrameIdxs [1], so the offsets they get (as
> returned by MFI->getObjectOffset) don't directly correspond to their
> actual stack locations.  In X86FrameLowering's
> emitCalleeSavedFrameMoves, when emitting DWARF information, this
> discrepancy gets fixed up by subtracting the offset reported by
> MFI->getObjectOffset from the minimum offset for any CSR (this is done
> by the "Offset = MaxOffset - Offset + saveAreaOffset;" line).  Is
> there a reason why llvm doesn't keep around the offsets in the right
> order from very beginning, by pushing the CSRs in increasing order of
> FrameIdxs?

Now, that you mention it, I remember being down to the same rabbit hole. 
With certain calling conventions (coldcc, I think it was which can for 
sure expose this for x86), it is possible to generate invalid CFI 
directives for the registers in a frame. Especially when XMM registers 
must be preserved along with general purpose registers. And the reason 
for this was the offset fixing logic within emitCalleeSavedFrameMoves, 
which breaks when fixing offset for XMM registers.

To fix this disparity, I concluded that it could be done by reversing 
definition order of general purpose registers within X86CallingConv.td 
for all calling conventions, since llvm prefers to use push/pop model 
for storing GPR:s (for x86). With this change stack slots and registers 
would have 1:1 mapping, without extra offset calculations and 
emitCalleeSavedFrameMoves could be simplified by removing extra magic to 
determine slots, and to generate correct CFI directives in unusual cases.

> [1]: in fact, the way X86FrameLowering's spillCalleeSavedRegisters and
> PEI's calculateCalleeSavedRegisters are set up, I don't see a reason
> why the FrameIdxs and the generated push instructions have any
> relation at all.  It seems that the code relies on
> MFI->CreateStackObject returning sequential integers.
>
> Thanks!
> -- Sanjoy
>
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