[llvm-dev] About detailed rule of fastcall

[Reid Kleckner via llvm-dev]

1. probably for GCC compatibility
2. probably the same
3. Hard to say, this could be a bug, compare with GCC
4. This behavior was implemented in this change:
https://reviews.llvm.org/rC166538 You can find the logic for this in clang
today.

I also noticed that fastcall behaves differently when we target MSVC
compatibility.



On Fri, Sep 20, 2019 at 3:29 AM jy ban via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hello.
> I'm trying to understand how fastcall works.
>
> As far as I know, first two integer type arguments are passed in ecx and
> edx.
> I tested several test, but the result was different from what I expected
>
> #1
>     typedef struct _data_t {
>         int d;
>     } data_t;
>
>     void __attribute__((fastcall)) test_suuu(data_t s, unsigned int a,
> unsigned int b, unsigned int c);
>
> unsigned int a is passed in edx, and the rest are passed in stack (in s,
> b, c order).
>
> #2 : order of arguments is diff from #1
>    void __attribute__((fastcall)) test_usuu(unsigned int a, data_t s,
> unsigned int b, unsigned int c);
>
> unsigned int a is passed in ecx, and the rest are passed in stack (in s,
> b, c order)
>
> #3 : __attribute__((__packed__)) is diff from #1
>     typedef struct __attribute__((__packed__)) _data_t3 {
>         int d;
>     } data_t3;
>
>     void __attribute__((fastcall)) test_s3uu(data_t3 s3, unsigned int a,
> unsigned int b);
>
> unsigned int a is passed in ecx, and the rest are passed in stack (in s,
> b, c order)
>
> My questions are as follow
> - why struct s in first test is passed in edx not ecx?
> - why unsigned b is in second test is passed in stack not edx?
> - why 'packed' makes a difference?
> - where can I find relevant code in llvm?
>
> I'm using i586, linux, llvm 5.02
> Build with "clang++ -std=c++11 -ggdb -O0 -gdwarf-4 -fno-omit-frame-pointer
> -fno-optimize-sibling-calls -o ref_test ref_test.cpp"
> (All arguments are used in the function so that they are not optimized out)
> Tested with gdb
>     (gdb) break* test_suuu
>     (gdb) x/6wx $esp
>     (gdb) i r
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