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From: "etienne_lorrain at yahoo dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/53507] New: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag
Date: Mon, 28 May 2012 10:03:00 -0000
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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