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?
Thanks in advance.
--
Best Regards.
