[Bug target/54829] bad optimization: sub followed by cmp w/ zero (x86 & ARM)

[daniel.santos at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54829



Daniel Santos <daniel.santos at pobox dot com> changed:



           What    |Removed                     |Added

----------------------------------------------------------------------------

            Summary|bad optimization: sub       |bad optimization: sub

                   |followed by cmp w/ zero     |followed by cmp w/ zero

                   |(ARM)                       |(x86 & ARM)



--- Comment #3 from Daniel Santos <daniel.santos at pobox dot com> 2012-10-06 
15:55:47 UTC ---

Sure it can:



extern print_gt(void);

extern print_lt(void);

extern print_eq(void);



void cmp_and_branch(long a, long b)

{

    if (a > b) {

        print_gt();

    } else if (a < b) {

        print_lt();

    } else {

        print_eq();

    }

}



Here's x86_64:



cmp_and_branch:

.LFB0:

    .cfi_startproc

    cmpq    %rsi, %rdi

    jg  .L5

    jl  .L6

    jmp print_eq

    .p2align 4,,10

    .p2align 3

.L6:

    jmp print_lt

    .p2align 4,,10

    .p2align 3

.L5:

    jmp print_gt

    .cfi_endproc



And here's i386:



cmp_and_branch:

.LFB0:

    .cfi_startproc

    movl    8(%esp), %eax

    cmpl    %eax, 4(%esp)

    jg  .L5

    jl  .L6

    jmp print_eq

    .p2align 2,,3

.L6:

    jmp print_lt

    .p2align 2,,3

.L5:

    jmp print_gt

    .cfi_endproc