[LLVMdev] Handling SRet on Windows x86

[Timur Iskhodzhanov]

On Tue, Oct 2, 2012 at 8:28 PM, David Blaikie <dblaikie at gmail.com> wrote:
> On Tue, Oct 2, 2012 at 8:54 AM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
>> [+cfe-dev as this does seem like both LLVM+Clang issue]
>> [Sorry for an incomplete e-mail context, please see
>> http://llvm.org/PR13676#c6 if you're interested]
>>
>> I've read these bugs and now I'm even more confused than I was before :)
>>
>> What do you think about the following approach:
>> a) I'll create test cases for the major issues I've observed so far
>>    These test cases will check both -emit-llvm and llc output
>
> Just an aside: generally Clang tests should just verify the emitted
> LLVM bitcode.
I know about that, see below.

> If you want to test what machine code/assembly that
> compiles down to, that should be an LLVM test that starts at LLVM
> bitcode and goes down to machine code/assembly.
Are there any serious reasons not to do combined -emit-llvm+llc tests
for such issues that need both LLVM and Clang support?
I understand usually it's easy to split but in this particular case it
seems like the generated bitcode might need to be changed during Clang
patching.
In this particular case I believe having a combined test is much more
convenient.
WDYT?

>>    They'll have CHECKs for stuff that already works and
>> FIXME+CHECK-NOT for stuff that doesn't.
>>  I guess I should put these tests in clang/test/CodeGen[CXX] ?
>>
>> b) As a short-term solution to avoid blocking progress for those who
>> are interested in a functioning Windows compiler I'll publish my patch
>> [which breaks the non-Windows compatibility but improves Windows
>> compat] in PR13676.
>>
>> c) Having these test cases at hand, we can come up with a decent
>> long-term solution
>>
>> Does that sound good to you?
>>
>> On Tue, Oct 2, 2012 at 7:31 PM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
>>> On Tue, Oct 2, 2012 at 7:03 PM, Anton Korobeynikov <asl at math.spbu.ru> wrote:
>>>> Hello Timur,
>>>>
>>>>> I'd like to ask for advice:
>>>>> a) Is it OK to change the SRet/ThisCall behaviour on non-Windows platforms?
>>>>>     [I suppose no]
>>>> no
>>>>
>>>>>
>>>>> b) Should I be altering CC_X86_32_ThisCall
>>>>>     OR should I introduce CC_X86_Win32_ThisCall instead?
>>>>>     [Answer not clear to me - is there any platform besides Windows
>>>>>     that uses thiscall?]
>>>> no
>>> Can you please clarify which question you've answered here?
>>> Sorry for making the ambiguous questions in the first place :)
>>>
>>>> It seems for me that you're trying to solve the problem from the wrong
>>>> end. As far as I remember, there is a difference - "simple" (probable
>>>> POD-like stuff) are returned in the regs, while classes with
>>>> non-trivial ctors, etc. are passed / returned on stack.
>>> Sort of.
>>>
>>>> It's frontend responsibility to emit proper IR in this case.
>>> Isn't it what's SRet is supposed to be?
>>>
>>>> See http://llvm.org/bugs/show_bug.cgi?id=5064 and around. This seems
>>>> to be the correct description of what's going on.
>>> FTR, http://llvm.org/bugs/show_bug.cgi?id=5058 seems to be more up-to-date.
>>>
>>> Thanks for your reply!
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