[Bug rtl-optimization/21150] New: Suboptimal byte extraction from larger integers

[Bug rtl-optimization/21150] New: Suboptimal byte extraction from larger integers

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

Bytes are typically extracted from e.g. u64's by something like

#define D5(v) (((v) >> 40) & 0xff)

Testcase shows that gcc does not optimize this "good enough".

-- 
           Summary: Suboptimal byte extraction from larger integers
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vda at port dot imtp dot ilyichevsk dot odessa dot ua
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i386-pc-linux-gnu
  GCC host triplet: i386-pc-linux-gnu
GCC target triplet: i386-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150

[Bug rtl-optimization/21150] Suboptimal byte extraction from larger integers

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

------- Additional Comments From vda at port dot imtp dot ilyichevsk dot odessa dot ua  2005-04-21 13:12 -------
Created an attachment (id=8701)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=8701&action=view)
generate assembly with -S and compare results


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150

[Bug rtl-optimization/21150] Suboptimal byte extraction from 64bits

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
            Summary|Suboptimal byte extraction  |Suboptimal byte extraction
                   |from larger integers        |from 64bits


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150

[Bug rtl-optimization/21150] Suboptimal byte extraction from 64bits

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

------- Additional Comments From vda at port dot imtp dot ilyichevsk dot odessa dot ua  2005-04-24 13:26 -------
I don't think that bug description is correct.
I believe similar observation will be valid for byte extraction
from u32 and u16, and for u16-from-u32, etc.

Update for latest gcc.
This is what 4.0.0 produces from the testcase:

# gcc -O2 -fomit-frame-pointer -S helper.c
# cat helper.s
 [I removed non-essential stuff]
a:
        movl    v+8, %eax
        shrl    $8, %eax
        xorb    v, %al
        xorb    v+18, %al
        xorb    v+27, %al
        xorb    v+36, %al
        movl    v+40, %edx
        movl    v+44, %ecx
        movl    %ecx, %edx
        xorl    %ecx, %ecx
        shrl    $8, %edx
        xorl    %edx, %eax
        xorb    v+54, %al
        xorb    v+63, %al
        movzbl  %al, %eax
        ret
b:
        movl    v+8, %eax
        movl    v+12, %edx
        shrdl   $8, %edx, %eax
        shrl    $8, %edx
        xorb    v, %al
        movl    v+16, %edx
        movl    v+20, %ecx
        shrdl   $16, %ecx, %edx
        shrl    $16, %ecx
        xorl    %edx, %eax
        movl    v+24, %edx
        movl    v+28, %ecx
        shrdl   $24, %ecx, %edx
        shrl    $24, %ecx
        xorl    %edx, %eax
        xorb    v+36, %al
        movl    v+40, %edx
        movl    v+44, %ecx
        movl    %ecx, %edx
        xorl    %ecx, %ecx
        shrl    $8, %edx
        xorl    %edx, %eax
        xorb    v+54, %al
        xorb    v+63, %al
        movzbl  %al, %eax
        ret
c:
        movb    v+9, %al
        xorb    v, %al
        xorb    v+18, %al
        xorb    v+27, %al
        xorb    v+36, %al
        xorb    v+45, %al
        xorb    v+54, %al
        xorb    v+63, %al
        movzbl  %al, %eax
        ret
d:
        movl    v+8, %eax
        movl    v+12, %edx
        shrdl   $8, %edx, %eax
        shrl    $8, %edx
        xorb    v, %al
        movl    v+16, %edx
        movl    v+20, %ecx
        shrdl   $16, %ecx, %edx
        shrl    $16, %ecx
        xorl    %edx, %eax
        movl    v+24, %edx
        movl    v+28, %ecx
        shrdl   $24, %ecx, %edx
        shrl    $24, %ecx
        xorl    %edx, %eax
        xorb    v+36, %al
        movl    v+40, %edx
        movl    v+44, %ecx
        movl    %ecx, %edx
        xorl    %ecx, %ecx
        shrl    $8, %edx
        xorl    %edx, %eax
        xorb    v+54, %al
        xorb    v+63, %al
        movzbl  %al, %eax
        ret

As you can see, a,b and d results are far from optimal,
while c is almost perfect.

Note that people typically use d, i.e. this:
#define D7(v) (((v) >> 56))
#define D6(v) (((v) >> 48) & 0xff)
#define D5(v) (((v) >> 40) & 0xff)
#define D4(v) (((v) >> 32) & 0xff)
#define D3(v) (((v) >> 24) & 0xff)
#define D2(v) (((v) >> 16) & 0xff)
#define D1(v) (((v) >>  8) & 0xff)
#define D0(v) ((v) & 0xff)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150

[Bug rtl-optimization/21150] Suboptimal byte extraction from 64bits

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-04 22:21 -------
Confirmed, this is most likely a sub register problem.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-07-04 22:21:33
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150