https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88231
--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
The feature already exists at -Os by default (i.e., all functions are by
default minimally aligned). The suggestion here is only to let GCC minimize
the amount of padding it adds to functions in order to align the explicitly
overaligned ones that follow by changing the order it emits them in.
Outside -Os, functions would continue to be optimally aligned unless overridden
by the attribute. When their alignment is explicitly reduced by the attribute
GCC could still be smart about ordering them so as to minimize wasted space.
Consider:
__attribute__ ((aligned (4))) int f4 (int i) { return 2 * i; }
double f (double x) { return x * x * x; }
__attribute__ ((aligned (4))) int g4 (int i) { return i; }
for which GCC for x86_64 emits:
0000000000000000 <f4>: ;; unnecessarily overaligned
0: 8d 04 3f lea (%rdi,%rdi,1),%eax
3: c3 retq
4: 66 90 xchg %ax,%ax
6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
d: 00 00 00
0000000000000010 <f>: ;; optimally aligned
10: 66 0f 28 c8 movapd %xmm0,%xmm1
14: f2 0f 59 c8 mulsd %xmm0,%xmm1
18: f2 0f 59 c1 mulsd %xmm1,%xmm0
1c: c3 retq
1d: 0f 1f 00 nopl (%rax)
0000000000000020 <g4>: ;; also unnecessarily overaligned
20: 89 f8 mov %edi,%eax
22: c3 retq
If it laid down f first instead it would be able to avoid padding f4:
0000000000000000 <f>:
0: 66 0f 28 c8 movapd %xmm0,%xmm1
4: f2 0f 59 c8 mulsd %xmm0,%xmm1
8: f2 0f 59 c1 mulsd %xmm1,%xmm0
c: c3 retq
d: 0f 1f 00 nopl (%rax)
0000000000000010 <f4>: ;; unavoidably overaligned
10: 8d 04 3f lea (%rdi,%rdi,1),%eax
13: c3 retq
0000000000000014 <g4>: ;; aligned exactly as requested
14: 89 f8 mov %edi,%eax
16: c3 retq
This is probably not important outside -Os, but if it's implemented for -Os it
won't cost anything to also enable it at other optimization levels.
The following discussion provides some context:
https://gcc.gnu.org/ml/gcc/2018-11/msg00127.html
