[LLVMdev] questions about byval argument passing

Akira Hatanaka ahatanak at gmail.com
Fri Oct 21 09:37:02 PDT 2011

Thanks, I will try that.

On Fri, Oct 21, 2011 at 9:24 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Thu, Oct 20, 2011 at 2:21 PM, Akira Hatanaka <ahatanak at gmail.com> wrote:
>> Suppose this structure is passed by value to a function foo(struct S1),
>> struct S1 {
>>  int i0;
>>  float f0;
>>  double d0;
>>  long long l0;
>> };
>> and the ABI dictates the elements of the structure are copied to both
>> integer and floating pointer argument registers in the following way:
>> (i0, f0) => 1st 64-bit integer argument register
>> d0 => 2nd double float argument register (doubles are passed in FP registers)
>> l0 => 3rd 64-bit integer argument register
>> What is the best way to find out whether integer registers or floating
>> pointer registers should be used for argument passing inside
>> TargetLowering::LowerCall? I can get basic information of the
>> structure being passed (for example, by calling
>> ArgFlagsTy::getByValSize() or ArgFlagsTy::getByValAlign() to get size
>> and alignment), but there doesn't seem to be an easy way to find more
>> detailed information. Do I have to get the instance of StructType for
>> structure S1 and analyze it?
> The way we do stuff like this for x86-64 is adding handling to clang;
> for example, to handle your case, make clang lower "void f(struct S1
> x)" to the following:
> define void @x(i64 %x.1, double %x.2, i64 %x.3)
> Basically, an LLVM struct type doesn't really encode enough
> information to accurately compute the C calling convention, so we
> handle the tricky cases in the frontend.   The relevant code is in
> clang/lib/CodeGen/TargetInfo.cpp .
> -Eli

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