Fix PR15293: ARM codegen ice - expected larger existing stack allocation

[Manman Ren]

On Mar 29, 2013, at 11:05 AM, Stepan Dyatkovskiy wrote:

> Hello,
> 
> Below some results of my small investigation.
> 
> [code]
> target triple = "arm--linux-gnueabihf"
> define void @t(i32 %a, %struct.s* byval %s, ...) nounwind {
> define void @caller() {
>  call void (i32, %struct.s*, ...)* @t(i32 0, %struct.s* @v, i32 777);
>  ret void
> }
This is an incomplete paste. Could you give the full example?
> [/code]
> 
> llc generates wrong asm code for IR above.
> 
> After some simple tests I conclude the next:
> 
> X86: tries don't use "byval" IR attribute in all cases its possible: it replaces structures with set of smaller params, does bitcasts and so on. But in some narrow set of cases it emits "byval" attributes.
> 
> ARM:
>    clang: just casts structures to arrays and doesn't use byval at all.
>    IR: generates bad code when two byval params passed, or
>    "byval" + "..." pair passed. The only case works fine here is
>    the big byval structure that splitted onto reg-part and stack-part.

Do you have some sample codes for two byval params?

Thanks,
Manman
> 
> That because CCState assumes that only parameter could be passed by value.
> 
> I propose to replace the term of "FirstByValRegister" with "SmallVector<std::pair<StartReg, RegsCount> ByValInRegArgs". The last one allows to track several small structures passed by value; it also automatically solves problem when we have single tiny "byval" structure (PR15293).
> 
> And BTW since arm-linux-gnueabi-clang avoids byval IR attribute, this feature will not be activated after llvm will fixed. Or I'm wrong? Are there some cases when arm-clang still emits something with "byval" attribute?
> If no - byval structures handling shouldn't change clang behaviour.
> 
> Well, if I properly read the code, there are 3 points to be fixed in LLVM:
> 
> Callee side:
> 1. ARMTargetLowering::HandleByVal, since it allocates stack areas for arguments passed by value. Its also "caller" side though: for caller it just mark necessary amount of regs as byval.
> 2. ARMTargetLowering::LowerFormalArguments. Both for byval args and for var-arg function it calls VarArgStyleRegisters that allocates stack-frames for them and stores stores registers that contains byval arguments. Note if both byval parameter and "..." are presented it will store *the same* registers into all stack frames.
> 
> Caller side:
> 1. ARMTargetLowering::LowerCall. This funtion saves byval arguments into registers. If argument is too big to be stored in regs, method splits it onto reg-part and stack-part.
> 
> -Stepan.
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