https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113838
Bug ID: 113838
Summary: regression of redundant load operation introduced by
-fno-tree-forwprop introduce
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: absoler at smail dot nju.edu.cn
Target Milestone: ---
hi, I have found for the following code, with -O2 option, gcc-10.2.0 will
generate a redundant load, and gcc-13.2.0 won't. However, with an extra flag "
-fno-tree-forwprop", gcc-13.2.0 will produce the same bad code
full code https://godbolt.org/z/objsWGnY6
```
func_37() ...
l_50[1] = &g_51;
if (((*l_50[1]) ^= g_26[5][3][0]))
{ /* block id: 13 */
int32_t **l_52[5] = {&l_50[2],&l_50[2],&l_50[2],&l_50[2],&l_50[2]};
int i;
(*l_52[4]) = (((void*)0 != &g_51) , &g_36[3][4]);
return p_39;
}
else
{ /* block id: 16 */
int32_t *l_53 = &g_54[0][1][1];
return l_53;
}
```
```
func_37():
401d74: mov 0x364e(%rip),%edx # 4053c8 <g_26+0x1a8>
func_11():
401d7a: mov %al,0x33d9(%rip) # 405159 <g_44+0x9>
func_37():
401d80: mov 0x33c2(%rip),%eax # 405148 <g_51>
401d86: xor %eax,%edx
401d88: cmp 0x363a(%rip),%eax # 4053c8 <g_26+0x1a8>
401d8e: mov $0x40510c,%eax
401d93: mov %edx,0x33af(%rip) # 405148 <g_51>
401d99: mov $0x4051a4,%edx
401d9e: cmovne %rdx,%rax
```
the second load of g_26[5][3][0], i.e. "cmp 0x363a(%rip),%eax" can be
optimized away. The better code generated by gcc-13.2.0 is:
```
func_37():
401e40: mov 0x3582(%rip),%eax # 4053c8 <g_26+0x1a8>
401e46: mov %edx,%ecx
401e48: xor %eax,%ecx
401e4a: cmp %eax,%edx
401e4c: mov $0x40510c,%edx
401e51: mov $0x4051a4,%eax
401e56: cmove %rdx,%rax
```
