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From: "redbeard0531 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/99470] New: Convert fixed index addition to array address
 offset
Date: Mon, 08 Mar 2021 17:20:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99470

            Bug ID: 99470
           Summary: Convert fixed index addition to array address offset
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redbeard0531 at gmail dot com
  Target Milestone: ---

These two functions do the same thing but f() is the cleaner source code
(especially when arr is a std::array) while g() generates better code:

https://gcc.godbolt.org/z/vTT399

#include <cstdint>

inline int8_t arr[256];

bool f(int a, int b) {
    return arr[a+128] =3D=3D arr[b+128];
}

bool g(int a, int b) {
    return (arr+128)[a] =3D=3D (arr+128)[b];
}

f(int, int):
        sub     edi, -128
        sub     esi, -128
        lea     rax, arr[rip]
        movsx   rdi, edi
        movsx   rsi, esi
        movzx   edx, BYTE PTR [rax+rsi]
        cmp     BYTE PTR [rax+rdi], dl
        sete    al
        ret
g(int, int):
        lea     rax, arr[rip+128]
        movsx   rdi, edi
        movsx   rsi, esi
        movzx   edx, BYTE PTR [rax+rsi]
        cmp     BYTE PTR [rax+rdi], dl
        sete    al
        ret

In addition to only doing the +128 once, it also ends up being completely f=
ree
in g() because the assembler (or linker?) folds the addition into the addre=
ss
calculation by adjusting the offset of the rip-relative address. In the god=
bolt
link, you can see that when compiled to binary, the LEA instruction uses the
same form in both f() and g(), so the addition really is free in g().=