GCC Bugzilla Bug 33315

if ( x == 8 ) statement1
if ( x != 8 ) statement1
if ( x == 9 ) statement2
if ( x != 9 ) statement2
should be replaced by

statement1
statement2

However this doesnt happen and compare and jumps do get generated.

Created an attachment (id=14158) [edit]
Sample Testcase

This is related to PR32306 and to the fact that we don't have a code hoisting
pass.  And related to PR30905 because cross-jumping fixes this up on the
rtl-level for both gcc 4.1 and 4.2:

test:
        pushl   %ebp
        movl    %esp, %ebp
        popl    %ebp
        movl    $0, a
        movl    $0, a+4
        movl    $0, a+8
        ret

As a missed optimization, this bug adds new information.  But as a regression,
this is a dup of bug 30905.

Is the missed optimization in the following code the same?

volatile unsigned char *reg_a = (unsigned char *)42;
volatile unsigned char *reg_b = (unsigned char *)34;

extern void a(void);
extern void b(void);
extern void c(void);

void
decide(void)
{
        signed char diff;

        diff = *reg_a - *reg_b;

        if (diff < 0)
                a();
        else if (diff == 0)
                b();
        else if (diff > 0)
                c();
}

The third "if" statement is partially executed: it apparently remembered
that diff could not be less than 0, but it still tests against 0 even
though that test has just been done before.  Verified on both, the AVR
and i386 target.

Interestingly, by just reordering the code, the third condition will
be eliminated by GCC 4.x (but not by GCC 3.x):

volatile unsigned char *reg_a = (unsigned char *)42;
volatile unsigned char *reg_b = (unsigned char *)34;

extern void a(void);
extern void b(void);
extern void c(void);

void
decide(void)
{
        signed char diff;

        diff = *reg_a - *reg_b;

        if (diff < 0)
                a();
        else if (diff > 0)
                c();
        else if (diff == 0)
                b();
}

If someone thinks that's an entirely different thing than the subject of this
bug, please tell me so, and I'll submit a separate one.

Subject: RE:  If condition not getting eliminated


Hi Ramana,

Please could you add sdkteam-gnu@icerasemi.com - then we all get to see
it ;-)

Cheers,


-----Original Message-----
From: ramana at icerasemi dot com [mailto:gcc-bugzilla@gcc.gnu.org] 
Sent: 21 October 2008 08:04
To: Dave Edwards
Subject: [Bug middle-end/33315] If condition not getting eliminated



-- 

ramana at icerasemi dot com changed:

           What    |Removed                     |Added
------------------------------------------------------------------------
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                 CC|                            |ramana at icerasemi dot
com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33315

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