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

["daniel.santos at pobox dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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

             Bug #: 54829
           Summary: bad optimization: sub followed by cmp w/ zero (x86 &
                    ARM)
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: daniel.santos@pobox.com


I originally posted this under bug #3507 but have since discovered that it is
target-specific and is a separate issue than bug #3507.

extern print_gt(void);
extern print_lt(void);
extern print_eq(void);

void cmp_and_branch(long a, long b)
{
    long diff = a - b;

    if (diff > 0) {
        print_gt();
    } else if (diff < 0) {
        print_lt();
    } else {
        print_eq();
    }
}

Here, result of the subtraction is directly used in the branch code and nowhere
else.  However, gcc -O2 -S still generates this output:

cmp_and_branch:
.LFB0:
    .cfi_startproc
    subq    %rsi, %rdi
    cmpq    $0, %rdi
    jg    .L5
    jne    .L6
    jmp    print_eq
    .p2align 4,,10
    .p2align 3
.L5:
    jmp    print_gt
    .p2align 4,,10
    .p2align 3
.L6:
    jmp    print_lt
    .cfi_endproc

Notice that we're using subq followed by cmpq instead of just cmpq %rsi, %rdi. 
In another case, where there is a loop and one of the values compared against
remains the same, an additional mov instruction is required to prevent the
unchanging value's register from being destroyed, so it actually generates two
extra instructions in that situation.

When built on ARM, we get something similar:
cmp_and_branch:
    @ args = 0, pretend = 0, frame = 0
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    rsb    r1, r1, r0
    cmp    r1, #0
    bgt    .L5
    bne    .L6
    b    print_eq
.L5:
    b    print_gt
.L6:
    b    print_lt

Note here that we do rsb followed by cmp with zero again.  However, on PPC
(apinski from freenode compiled this for me), the result is actually correct:

subf. 9,4,3
bgt 0,.L5
bne 0,.L6
print_eq

Finally, on MIPS (also from apinski):
dsubu $4,$4,$5
bgtz $4,$L5
nop