[Bug optimization/14617] New: [tree-ssa] suboptimal code ('0' <= c && c <= '9') ? c - '0' : 0

["kazu at cs dot umass dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

Here is what I get with the current tree-ssa:

int
foo (char c)
{
  return ('0' <= c && c <= '9') ? c - '0' : 0;
}
/*
	movzbl	4(%esp), %ecx
	xorl	%eax, %eax
	movb	%cl, %dl
	subb	$48, %dl
	cmpb	$9, %dl
	ja	.L4
	movsbl	%cl,%eax      ; extending again (but this time sign-ext)?
	subl	$48, %eax     ; again subtracting 48 (but in different mode)?
.L4:
	ret
*/

int
bar (char c)
{
  c -= '0';
  return ((unsigned char) c <= 9) ? c : 0;
}
/*
	movzbl	4(%esp), %ecx
	xorl	%eax, %eax
	subb	$48, %cl
	movsbl	%cl,%edx      ; extending again (but this time sign-ext)?
	cmpb	$10, %cl
	cmovb	%edx, %eax
*/

int
baz (char c)
{
  int tem = c;
  tem -= '0';
  return ((unsigned char) tem <= 9) ? tem : 0;
}
/*
	movsbl	4(%esp), %edx
	xorl	%eax, %eax
	subl	$48, %edx
	cmpb	$10, %dl
	cmovb	%edx, %eax
*/

For "foo", here is what I get right before TER:

foo (c)
{
  unsigned int tem;
  unsigned char T.6;
  int iftmp.5;
  unsigned char c.4;
  int T.3;
  unsigned char T.2;
  char T.1;
  int iftmp.0;

<bb 0>:
  T.1_3 = c_2 - 48;
  T.2_4 = (unsigned char)T.1_3;
  if (T.2_4 <= 9) goto <L0>; else goto <L2>;

<L0>:;
  T.3_7 = (int)c_2;
  iftmp.0_8 = T.3_7 - 48;

  # iftmp.0_1 = PHI <0(0), iftmp.0_8(1)>;
<L2>:;
  return iftmp.0_1;

}

Note that if we extended c_2 earlier and did all the computation in int
wherever possible, it would be pretty easy to avoid subtracting of 48 twice.
The idea of extending everything first comes from a paper

Effective Sign Extension Elimination
Kawahito, Komatsu, and Nakatani
http://www.trl.ibm.com/projects/jit/paper/sxt.pdf

-- 
           Summary: [tree-ssa] suboptimal code ('0' <= c && c <= '9') ? c -
                    '0' : 0
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: enhancement
          Priority: P2
         Component: 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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14617