[PATCH] fix for PR16393: miscompile with struct byval

[Stepan Dyatkovskiy]

Hi Manman,

After some learning what exactly happens. IMHO, your decision is the 
*only* way (that couldn't be called too hackish or insane :-)).

Though, may be we add SpAdj just as property for MachineBasicBlock class?

I've modified your patch, please find it in attachment.

-Stepan.

Manman Ren wrote:
>
> For a call site with struct byval, we will generate a sequence of instructions: STACKDOWN, STRUCT_BYVAL and STACKUP.
> STRUCT_BYVAL later on is expanded to a for loop. This caused issue in PEI when trying to eliminate frame indices.
>
> BB1:
>    STACKDOWN
> BB2:
>    memcpy
> BB3:
>    STACKUP
>
> PEI currently assumes SPAdj is 0 at the beginning of a basic block, but this is not true for BB2 or BB3. The proposed patch tries
> to fix the problem by inserting SET_SP_ADJ in BB2 and BB3:
> BB1:
>    STACKDOWN
> BB2:
>    SET_SP_ADJ
>    memcpy
> BB3:
>    SET_SP_ADJ
>    STACKUP
>
> A target-independent opcode SET_SP_ADJ is introduced, and pseudo instruction STRUCT_BYVAL is modified to take one extra
> argument (the amount of SP Adjustment). When we are expanding STRUCT_BYVAL to a for loop, we add SET_SP_ADJ to BB2 and BB3.
>
> PEI is also modified to handle SET_SP_ADJ. To make sure SET_SP_ADJ is always before any spill code that references frame indices,
> SET_SP_ADJ is treated as a label.
>
> SET_SP_ADJ can be useful for other targets when STACKDOWN and STACKUP are not paired up in the same basic block.
>
> Comments and other suggestions are welcome.
>
> Thanks,
> Manman
>

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