[LLVMdev] Structs passed by value

[Jan Voung]

Hi,

On Wed, Jun 13, 2012 at 9:55 AM, Martinez, Javier E <
javier.e.martinez at intel.com> wrote:

>  Hello,****
>
> ** **
>
> I’m trying to change the default behavior for how structures are passed to
> functions to use pass-by-value. Currently LLVM’s default behavior is to
> pass structures by reference. I’m not disputing the benefits of this but I
> really want to change the default behavior for experimentation purposes.
>


Are you sure that the current behavior is passing structures by reference?
 In your example, the function "convert" is given a pointer %in, but that
pointer is marked "byval".  According to http://llvm.org/docs/LangRef.html that
means that it is pass-by-value:

 "This indicates that the pointer parameter should really be passed by
value to the function. The attribute implies that a hidden copy of the
pointee is made between the caller and the callee, so the callee is unable
to modify the value in the caller."

etc...

Is the problem that in the backend, you do not see a copy being made?



****
>
> ** **
>
> To this end I’ve changed the code in
> DefaultABIInfo::classifyArgumentType() to use the different types. What
> I’ve found is that none of the options yield the expected results for the
> input code (see below). Except for the indirect case, the function
> signature is changed to take each structure element. How can I modify LLVM
> to pass structures by value?****
>
> ** **
>
> Thanks,****
>
> Javier****
>
> ** **
>
> [Original Code]****
>
> struct myType {****
>
>   long val;****
>
> };****
>
> int convert(struct myType in)****
>
> {****
>
>   return (int)in.val;****
>
> }****
>
> ** **
>
> [Expected Result]****
>
> define i32 @convert(%myType %in) nounwind readonly alwaysinline{****
>
> entry:****
>
>   %in.addr = alloca %myType, align 8****
>
>   store %myType %in, %myType* %in.addr, align 8****
>
>   %0 = getelementptr inbounds %myType* %in.addr, i32 0, i32 0****
>
>   %1 = load i64* %0, align 8****
>
>   %conv = trunc i64 %1 to i32****
>
>   ret i32 %conv****
>
> }****
>
> ** **
>
> [ABIArgInfo::getIndirect(0) – Default]****
>
> %struct.myType = type { i64 }****
>
> ** **
>
> define i32 @convert(%struct.myType* nocapture byval %in) nounwind readonly
> {****
>
> entry:****
>
>   %val = getelementptr inbounds %struct.myType* %in, i64 0, i32 0****
>
>   %0 = load i64* %val, align 8, !tbaa !0****
>
>   %conv = trunc i64 %0 to i32****
>
>   ret i32 %conv****
>
> }****
>
> ** **
>
> [ABIArgInfo::getExtend(), ABIArgInfo::getDirect(), ABIArgInfo::getExpand()]
> ****
>
> define i32 @convert(i64 %in.coerce0) nounwind readnone {****
>
> entry:****
>
>   %conv = trunc i64 %in.coerce0 to i32****
>
>   ret i32 %conv****
>
> }****
>
> ** **
>
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