https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116027
Bug ID: 116027
Summary: Redundant backup and restore of register with
-flive-range-shrinkage
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: user202729 at protonmail dot com
Target Milestone: ---
Code:
__attribute__((pure)) long f(long a, long b);
long x;
long g(){
long y=x;
long z=f(0, 0);
return f(y, z);
}
long h(){
long z=f(0, 0);
long y=x;
return f(y, z);
}
Resulting assembly:
g():
push rbx
xor esi, esi
xor edi, edi
call f(long, long)
mov rdi, QWORD PTR x[rip]
pop rbx
mov rsi, rax
jmp f(long, long)
h():
xor esi, esi
xor edi, edi
call f(long, long)
mov rdi, QWORD PTR x[rip]
mov rsi, rax
jmp f(long, long)
In function g, the push rbx/pop rbx pair is redundant.
Godbolt: https://godbolt.org/z/os4EYP916
Without -flive-range-shrinkage then both generates optimal code.
The reason for the suboptimal code can be partially traced to the following
code in gcc/ira.cc:
/* Don't move insns if live range shrinkage or register
pressure-sensitive scheduling were done because it will not
improve allocation but likely worsen insn scheduling. */
if (optimize
&& !flag_live_range_shrinkage
&& !(flag_sched_pressure && flag_schedule_insns))
combine_and_move_insns ();
Without the flag specified, the load of x is moved to after the function call
(which is optimal).
With the flag specified, the move is not done in that pass (but it is optimized
in a subsequent pass anyway, that pass is probably after the pass that removes
redundant stack backup/restores).
