question on inconsistent generated codes for builtin calls

[Richard Guenther]

On Fri, Jan 13, 2012 at 9:04 AM, Amker.Cheng <amker.cheng@gmail.com> wrote:
> Hi,
> I noticed gcc generates inconsistent codes for same function for builtin calls.
>
> compile following program:
> ----------------------------------------------
> #include <math.h>
> int a(float x) {
>      return sqrtf(x);
> }
> int b(float x) {
>      return sqrtf(x);
> }
>
> With command:
> arm-none-eabi-gcc -mthumb -mhard-float -mfpu=fpv4-sp-d16
> -mcpu=cortex-m4 -O0 -S a.c -o a.S
>
> The generated assembly codes is like:
> ----------------------------------------------
> a:
>        @ args = 0, pretend = 0, frame = 8
>        @ frame_needed = 1, uses_anonymous_args = 0
>        push    {r7, lr}
>        sub     sp, sp, #8
>        add     r7, sp, #0
>        fsts    s0, [r7, #4]
>        flds    s15, [r7, #4]
>        fsqrts  s15, s15
>        fcmps   s15, s15
>        fmstat
>        beq     .L2
>        flds    s0, [r7, #4]
>        bl      sqrtf
>        fcpys   s15, s0
> .L2:
>        ftosizs s15, s15
>        fmrs    r3, s15 @ int
>        mov     r0, r3
>        add     r7, r7, #8
>        mov     sp, r7
>        pop     {r7, pc}
>        .size   a, .-a
>        .align  2
>        .global b
>        .thumb
>        .thumb_func
>        .type   b, %function
> b:
>        @ args = 0, pretend = 0, frame = 8
>        @ frame_needed = 1, uses_anonymous_args = 0
>        push    {r7, lr}
>        sub     sp, sp, #8
>        add     r7, sp, #0
>        fsts    s0, [r7, #4]
>        flds    s0, [r7, #4]
>        bl      sqrtf
>        fcpys   s15, s0
>        ftosizs s15, s15
>        fmrs    r3, s15 @ int
>        mov     r0, r3
>        add     r7, r7, #8
>        mov     sp, r7
>        pop     {r7, pc}
>        .size   b, .-b
>
>
> The cause is in function expand_builtin, gcc checks following conditions:
> ----------------------------------------------
>  /* When not optimizing, generate calls to library functions for a certain
>     set of builtins.  */
>  if (!optimize
>      && !called_as_built_in (fndecl)
>      && DECL_ASSEMBLER_NAME_SET_P (fndecl)
>      && fcode != BUILT_IN_ALLOCA
>      && fcode != BUILT_IN_ALLOCA_WITH_ALIGN
>      && fcode != BUILT_IN_FREE)
>    return expand_call (exp, target, ignore);
>
> The control flow is:
> 1, DECL_ASSEMBLER_NAME_SET_P (fndecl) is false at the first time when
> compiling a;
> 2, It is then set in following codes when expanding sqrtf call in function a;
> 3, When compiling function b, gcc checks DECL_ASSEMBLER_NAME_SET_P (fndecl)
>     again and this time it's true;
>
> I am a little confused why we check DECL_ASSEMBLER_NAME_SET_P here.
> Does it have special meaning?

No, I think the check is superfluous and should be removed.  I also wonder
why we exempt BUILT_IN_FREE here ... can you dig in SVN history a bit?
For both things?

Thanks,
Richard.

> Thanks in advance.
>
> --
> Best Regards.