[Bug rtl-optimization/53507] New: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag

[Bug rtl-optimization/53507] New: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag

[etienne_lorrain at yahoo dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53507

             Bug #: 53507
           Summary: ia32/amd64: bsf can be used to test null memory, bsf
                    sets zero flag
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: etienne_lorrain@yahoo.fr


Compiling:
int I;
void ifnull(void);
void testnull(void) {
  if (I == 0)
    ifnull();
}

void fct (unsigned u) {
  if (u != 0)
    I = __builtin_ffs(u) - 1;
}

result in for testnull():
    movl    I, %eax
    testl    %eax, %eax
    je    .L4
    rep
    ret
.L4:
    jmp    ifnull()
It would be shorter/quicker to replace the two first lines by:
        bsfl     I, %eax
        je       ifnull()
i.e. there is no need to load the memory variable into a register with bsf/bsr.

And result for fct():
    movl    4(%esp), %eax
    testl    %eax, %eax
    je    .L5
    bsfl    %eax, %eax
    movl    %eax, I
.L5:
    rep
    ret
It would be shorter/quicker to test if the parameter u is null by the bsf
instruction:
    bsfl    4(%esp), %eax
    cmovne    %eax, I
.L5:
    rep
    ret

[Bug target/53507] ia32/amd64: bsf can be used to test null memory, bsf sets zero flag

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53507

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |target
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2021-11-29
             Status|UNCONFIRMED                 |NEW
             Target|                            |x86_64 i?86
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
LLVM does:
testnull():                           # @testnull()
        .cfi_startproc
# %bb.0:
        cmpl    $0, I
        je      .LBB0_2
# %bb.1:
        retl
.LBB0_2:
        jmp     ifnull()