https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107050
Bug ID: 107050
Summary: duplicate load of return value when facing multiple
branches
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: absoler at smail dot nju.edu.cn
Target Milestone: ---
given this code:
int g_286 = (-5L);
int p;
int f = 1;
void func_58();
func_31(int c, int d) {
if (c) {
if (f){
if (d)
func_58();
return g_286;
}
g_286 = 0;
}
return 0;
}
func_58() {
int arr[30];
p = arr[0];
}
when compiled with gcc-12.1.0 (-O1), it will generate:
0000000000401186 <func_58>:
401186: 48 83 ec 10 sub $0x10,%rsp
40118a: 8b 44 24 88 mov -0x78(%rsp),%eax
40118e: 89 05 f4 8c 00 00 mov %eax,0x8cf4(%rip) #
409e88 <p>
401194: 48 83 c4 10 add $0x10,%rsp
401198: c3 retq
0000000000401199 <func_31>:
401199: 89 f8 mov %edi,%eax
40119b: 85 ff test %edi,%edi
40119d: 74 1f je 4011be <func_31+0x25>
40119f: 8b 05 bb 2e 00 00 mov 0x2ebb(%rip),%eax #
404060 <f>
4011a5: 85 c0 test %eax,%eax
4011a7: 75 0b jne 4011b4 <func_31+0x1b>
4011a9: c7 05 b1 2e 00 00 00 movl $0x0,0x2eb1(%rip) #
404064 <g_286>
4011b0: 00 00 00
4011b3: c3 retq
4011b4: 8b 05 aa 2e 00 00 mov 0x2eaa(%rip),%eax #
404064 <g_286>
4011ba: 85 f6 test %esi,%esi
4011bc: 75 01 jne 4011bf <func_31+0x26>
4011be: c3 retq
4011bf: 48 83 ec 08 sub $0x8,%rsp
4011c3: b8 00 00 00 00 mov $0x0,%eax
4011c8: e8 b9 ff ff ff callq 401186 <func_58>
4011cd: 8b 05 91 2e 00 00 mov 0x2e91(%rip),%eax #
404064 <g_286>
4011d3: 48 83 c4 08 add $0x8,%rsp
4011d7: c3 retq
we can see in the func_31, compiler choose to load g_286 before judge whether d
!= 0, and if it's true, %eax will be used and func_58 is called. Before return,
g_286 will be loaded again to %eax
