[Bug rtl-optimization/17935] New: Two consecutive movzbl are generated

[Bug rtl-optimization/17935] New: Two consecutive movzbl are generated

[kazu at cs dot umass dot edu]

Consider:

struct flags {
  unsigned f0 : 1;
};

_Bool
bar (struct flags *p, struct flags *q)
{
  return (!p->f0 && !q->f0);
}

With "cc1 -O2 -fomit-frame-pointer -march=i386", I get

bar:
	movl	4(%esp), %eax
	testb	$1, (%eax)
	jne	.L9
	movl	8(%esp), %eax
	testb	$1, (%eax)
	sete	%al
	movzbl	%al, %eax
	movzbl	%al, %eax
	ret
	.p2align 2,,3
.L9:
	xorl	%eax, %eax
	movzbl	%al, %eax
	ret

Note the two consecutive movzbl.  We don't need the second one.

Also note the xorl followed by movzbl.  We don't need the movzbl.

-- 
           Summary: Two consecutive movzbl are generated
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] [4.0 Regression] Two consecutive movzbl are generated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-11 17:24 -------
Confirmed, this is 4.0 Regression again.
3.4.0 gives:
bar:
        movl    4(%esp), %eax
        xorl    %edx, %edx
        testb   $1, (%eax)
        jne     .L2
        movl    8(%esp), %ecx
        testb   $1, (%ecx)
        jne     .L2
        movb    $1, %dl
        .p2align 2,,3
.L2:
        movzbl  %dl, %eax
        ret

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|enhancement                 |minor
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
      Known to fail|                            |4.0.0
      Known to work|                            |3.4.0
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-11 17:24:44
               date|                            |
            Summary|Two consecutive movzbl are  |[4.0 Regression] Two
                   |generated                   |consecutive movzbl are
                   |                            |generated
   Target Milestone|---                         |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] Two consecutive movzbl are generated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-12 19:20 -------
Note this is not a really a regression, the main issue I see here is that we are ifcvt if(a) 1 else 0; to a and 
then we copy the return part (the movzbl and ret) on the other branch but we don't combine them.  
Maybe we should rerun combine after reload.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|3.4.0                       |
            Summary|[4.0 Regression] Two        |Two consecutive movzbl are
                   |consecutive movzbl are      |generated
                   |generated                   |
   Target Milestone|4.0.0                       |---


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] Two consecutive movzbl are generated

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-12-12 17:57 -------
With today's mainline, I get

bar:
	movl	4(%esp), %eax
	testb	$1, (%eax)
	jne	.L2
	movl	8(%esp), %eax
	testb	$1, (%eax)
	jne	.L2
	movl	$1, %eax
	movzbl	%al, %eax
	ret
	.p2align 2,,3
.L2:
	xorl	%eax, %eax
	movzbl	%al, %eax
	ret

Still we have a similar problem.  movl (or xorl) followed by movzbl is
meaningless.

It turns out that the fix for PR 14843 can fix this at tree level.
Here is the resulting assembly with my patch for PR 14843.

bar:
	movl	4(%esp), %eax
	testb	$1, (%eax)
	jne	.L2
	movl	8(%esp), %eax
	testb	$1, (%eax)
	jne	.L2
	movl	$1, %eax
	ret
	.p2align 2,,3
.L2:
	xorl	%eax, %eax
	ret

The reason my patch fixes this is because it removes casts before expansion.

Without my patch:

bar (p, q)
{
  int iftmp.0;

<bb 0>:
  if (p->f0 != 0) goto <L3>; else goto <L0>;

<L0>:;
  if (q->f0 != 0) goto <L3>; else goto <L7>;

<L7>:;
  iftmp.0 = 1;
  goto <bb 4> (<L8>);

<L3>:;
  iftmp.0 = 0;

<L8>:;
  return (int) (_Bool) iftmp.0;  <-- Notice these casts

}

With my patch:

bar (p, q)
{
  int D.1122;

<bb 0>:
  if (p->f0 != 0) goto <L3>; else goto <L0>;

<L0>:;
  if (q->f0 != 0) goto <L3>; else goto <L7>;

<L7>:;
  D.1122 = 1;
  goto <bb 4> (<L8>);

<L3>:;
  D.1122 = 0;

<L8>:;
  return D.1122;  <-- Casts are gone!

}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] Two consecutive movzbl are generated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-12-12 18:06 -------
This seems like someone should be calling fold (and fold should be changing the types back of the 
orginal tree):
(int) (_Bool) ((int) p->f0 == 0 && (int) q->f0 == 0)

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] Two consecutive movzbl are generated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-12 17:52 -------
With the patch for PR 21520, we will get better code.
Right now on the mainline we get:
bar:
        movl    4(%esp), %eax
        testb   $1, (%eax)
        jne     .L7
        movl    8(%esp), %eax
        movb    (%eax), %al
        andl    $1, %eax
        xorl    $1, %eax
        movzbl  %al, %eax
        ret
        .p2align 2,,3
.L7:
        xorl    %eax, %eax
        movzbl  %al, %eax
        ret

but after that patch we should be able to remove the extra movzbl in the second branch.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |21520


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] Two consecutive movzbl are generated

[dberlin at gcc dot gnu dot org]

-- 
Bug 17935 depends on bug 21520, which changed state.

Bug 21520 Summary: missing PRE opportunity with operand after operand
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21520

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935

[Bug rtl-optimization/17935] Two consecutive movzbl are generated

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2005-05-13 10:07 -------
I think there is another optimization opportunity regarding movzbl following andl.

Consider this part:

        movb    (%eax), %al        EAX = x...xbbbbbbbb
        andl    $1, %eax           EAX = 0...00000000b
        movzbl  %al, %eax          (not needed)
        ret

If the value of the constant to andl operator is less than 2 ^ bit width of the
register we would like to extend, then andl instruction inherently performs
zero-extension.

So if the xorl in comment #5 is moved before andl, we could apply above
simplification to get:

        movb    (%eax), %al
        xorl    $1, %eax
        andl    $1, %eax
        ret

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17935