Mike Frysinger wrote:
the pointer itself is the thing that is const, not the memory it points to.
this lets the compiler optimize the loads by generating relocations via the
pointer ... there's the fixup at the initial load time, but after that, it's
just offsets to a constant memory location. but removing the const markings,
the compiler has to reload the pointer everytime.
Actually, why does the comipler have to reload the pointer? In your
example, there is no other function called between accesses to the
pointer that might change the pointer, and anti-aliasing should let the
compiler know that the memory modified through the pointer can't be the
pointer itself because of the type difference.
you can check this yourself. this is the delta from removing the const:
54604 1222 488 56314 dbfa busybox.const
54964 1222 488 56674 dd62 busybox.no.const
+360 +360
and an example from the disassembly.
const/0000000000000000 <raise_exception>:
0: 48 83 ec 08 sub $0x8,%rsp
4: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # b
<raise_exception+0xb>
7: R_X86_64_PC32 ash_ptr_to_globals_misc-0x4
b: 8b 50 38 mov 0x38(%rax),%edx
e: ff c2 inc %edx
10: 89 50 38 mov %edx,0x38(%rax)
13: 40 88 78 3f mov %dil,0x3f(%rax)
17: 48 8b 78 30 mov 0x30(%rax),%rdi
1b: be 01 00 00 00 mov $0x1,%esi
20: e8 00 00 00 00 callq 25 <raise_exception+0x25>
21: R_X86_64_PC32 __longjmp_chk-0x4
no.const/0000000000000000 <raise_exception>:
0: 48 83 ec 08 sub $0x8,%rsp
4: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # b
<raise_exception+0xb>
7: R_X86_64_PC32 ash_ptr_to_globals_misc-0x4
b: 8b 42 38 mov 0x38(%rdx),%eax
e: ff c0 inc %eax
10: 89 42 38 mov %eax,0x38(%rdx)
extra load <<<
13: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 1a
<raise_exception+0x1a>
16: R_X86_64_PC32 ash_ptr_to_globals_misc-0x4
1a: be 01 00 00 00 mov $0x1,%esi
1f: 40 88 78 3f mov %dil,0x3f(%rax)
23: 48 8b 78 30 mov 0x30(%rax),%rdi
27: e8 00 00 00 00 callq 2c <raise_exception+0x2c>
28: R_X86_64_PC32 __longjmp_chk-0x4
would be nice if we documented all the little tricks we used somewhere.
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