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* [Bug pending/27971] New: eliminate shift in array[(x>>2)&3]
@ 2006-06-09 6:18 dean at arctic dot org
2006-06-09 6:20 ` [Bug target/27971] " pinskia at gcc dot gnu dot org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: dean at arctic dot org @ 2006-06-09 6:18 UTC (permalink / raw)
To: gcc-bugs
array[(x>>2)&3] can be arranged to avoid the right shift.
consider:
% cat shift-and-fold.c
unsigned array[4];
unsigned foo(unsigned long x)
{
return array[(x>>2)&3ul];
}
% /home/odo/gcc/bin/gcc -g -O3 -Wall -c -o shift-and-fold.o shift-and-fold.c
% objdump -dr shift-and-fold.o
shift-and-fold.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 48 c1 ef 02 shr $0x2,%rdi
4: 83 e7 03 and $0x3,%edi
7: 8b 04 bd 00 00 00 00 mov 0x0(,%rdi,4),%eax
a: R_X86_64_32S array
e: c3 retq
could be:
and $0xc,%edi
mov 0(%rdi),%eax
gcc does the right thing when the left shift isn't part of an address...
thanks
-dean
% /home/odo/gcc/bin/gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../gcc/configure --prefix=/home/odo/gcc
--host=x86_64-linux-gnu --disable-multilib --enable-languages=c,c++
--disable-bootstrap
Thread model: posix
gcc version 4.2.0 20060603 (experimental)
--
Summary: eliminate shift in array[(x>>2)&3]
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: pending
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dean at arctic dot org
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/27971] eliminate shift in array[(x>>2)&3]
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
@ 2006-06-09 6:20 ` pinskia at gcc dot gnu dot org
2006-06-09 6:43 ` pinskia at gcc dot gnu dot org
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-06-09 6:20 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2006-06-09 06:18 -------
This works correctly on powerpc. The rtl we get on x86_64 is:
(insn 10 7 11 2 (parallel [
(set (reg:DI 61)
(lshiftrt:DI (reg/v:DI 59 [ x ])
(const_int 2 [0x2])))
(clobber (reg:CC 17 flags))
]) 456 {*lshrdi3_1_rex64} (insn_list:REG_DEP_TRUE 6 (nil))
(expr_list:REG_DEAD (reg/v:DI 59 [ x ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 11 10 13 2 (parallel [
(set (reg:DI 62)
(and:DI (reg:DI 61)
(const_int 3 [0x3])))
(clobber (reg:CC 17 flags))
]) 297 {*anddi_1_rex64} (insn_list:REG_DEP_TRUE 10 (nil))
(expr_list:REG_DEAD (reg:DI 61)
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 13 11 17 2 (set (reg:SI 64)
(mem/s:SI (plus:DI (mult:DI (reg:DI 62)
(const_int 4 [0x4]))
(symbol_ref:DI ("array") <var_decl 0x2aaaaaf4bbb0 array>)) [3
array S4 A32])) 40 {*movsi_1} (insn_list:REG_DEP_TRUE 11 (nil))
(expr_list:REG_DEAD (reg:DI 62)
(nil)))
so I wonder if the something is not being reduced correctly.
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Component|pending |target
Ever Confirmed|0 |1
GCC host triplet|x86_64-linux-gnu |
Last reconfirmed|0000-00-00 00:00:00 |2006-06-09 06:18:37
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/27971] eliminate shift in array[(x>>2)&3]
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
2006-06-09 6:20 ` [Bug target/27971] " pinskia at gcc dot gnu dot org
@ 2006-06-09 6:43 ` pinskia at gcc dot gnu dot org
2007-11-28 23:06 ` raksit at google dot com
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-06-09 6:43 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2006-06-09 06:20 -------
Here is the RTL we get on PPC:
(insn 6 9 7 2 (set (reg/v:SI 120 [ x ])
(reg:SI 3 r3 [ x ])) 327 {*movsi_internal1} (nil)
(expr_list:REG_DEAD (reg:SI 3 r3 [ x ])
(expr_list:REG_EQUIV (mem/c/i:SI (plus:SI (reg/f:SI 67 ap)
(const_int 24 [0x18])) [4 x+0 S4 A32])
(nil))))
...
(insn 13 12 14 2 (set (reg:SI 124)
(lshiftrt:SI (reg/v:SI 120 [ x ])
(const_int 2 [0x2]))) 195 {lshrsi3_no_power}
(insn_list:REG_DEP_TRUE 6 (nil))
(expr_list:REG_DEAD (reg/v:SI 120 [ x ])
(nil)))
(insn 14 13 15 2 (parallel [
(set (reg:SI 125)
(and:SI (reg:SI 124)
(const_int 3 [0x3])))
(clobber (scratch:CC))
]) 146 {andsi3} (insn_list:REG_DEP_TRUE 13 (nil))
(expr_list:REG_DEAD (reg:SI 124)
(expr_list:REG_UNUSED (scratch:CC)
(nil))))
(insn 15 14 17 2 (set (reg:SI 126)
(ashift:SI (reg:SI 125)
(const_int 2 [0x2]))) 189 {ashlsi3_no_power}
(insn_list:REG_DEP_TRUE 14 (nil))
(expr_list:REG_DEAD (reg:SI 125)
(nil)))
Which gets reduced to:
(insn 15 14 17 2 (parallel [
(set (reg:SI 126)
(and:SI (reg:SI 3 r3 [ x ])
(const_int 12 [0xc])))
(clobber (scratch:CC))
]) 146 {andsi3} (nil)
(expr_list:REG_UNUSED (scratch:CC)
(expr_list:REG_DEAD (reg:SI 3 r3 [ x ])
(nil))))
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/27971] eliminate shift in array[(x>>2)&3]
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
2006-06-09 6:20 ` [Bug target/27971] " pinskia at gcc dot gnu dot org
2006-06-09 6:43 ` pinskia at gcc dot gnu dot org
@ 2007-11-28 23:06 ` raksit at google dot com
2007-12-05 19:28 ` raksit at google dot com
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: raksit at google dot com @ 2007-11-28 23:06 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from raksit at google dot com 2007-11-28 23:06 -------
I am looking into this.
--
raksit at google dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |raksit at google dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/27971] eliminate shift in array[(x>>2)&3]
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
` (2 preceding siblings ...)
2007-11-28 23:06 ` raksit at google dot com
@ 2007-12-05 19:28 ` raksit at google dot com
2008-01-11 4:51 ` raksit at gcc dot gnu dot org
2008-01-14 19:57 ` raksit at gcc dot gnu dot org
5 siblings, 0 replies; 7+ messages in thread
From: raksit at google dot com @ 2007-12-05 19:28 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from raksit at google dot com 2007-12-05 19:27 -------
For the rtl emitted on x86 processors, the combiner is almost able to optimize
the shift away. It combines and simplifies the 3 instructions down to:
Failed to match this instruction:
(set (reg:SI 64)
(mem/s:SI (plus:SI (and:SI (reg/v:SI 59 [ x ])
(const_int 12 [0xc]))
(symbol_ref:SI ("array") <var_decl 0xf7ef20b0 array>)) [3
array S4 A32]))
It gives up at this point. The solution is to introduce a split to do the AND,
followed by the load. This can be done in machine-independent way by modifying
find_split_point in combine.c. Patch coming up shortly.
-raksit
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/27971] eliminate shift in array[(x>>2)&3]
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
` (3 preceding siblings ...)
2007-12-05 19:28 ` raksit at google dot com
@ 2008-01-11 4:51 ` raksit at gcc dot gnu dot org
2008-01-14 19:57 ` raksit at gcc dot gnu dot org
5 siblings, 0 replies; 7+ messages in thread
From: raksit at gcc dot gnu dot org @ 2008-01-11 4:51 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from raksit at gcc dot gnu dot org 2008-01-11 04:21 -------
Subject: Bug 27971
Author: raksit
Date: Fri Jan 11 04:20:32 2008
New Revision: 131460
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=131460
Log:
gcc/ChangeLog
PR rtl-optimization/27971
* combine.c (find_split_point): introduced a new split for certain
types of mem rtx.
gcc/testsuite/ChangeLog
PR rtl-optimization/27971
* gcc.target/i386/pr27971.c: New testcase.
Added:
trunk/gcc/testsuite/gcc.target/i386/pr27971.c
Modified:
trunk/gcc/ChangeLog
trunk/gcc/combine.c
trunk/gcc/testsuite/ChangeLog
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/27971] eliminate shift in array[(x>>2)&3]
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
` (4 preceding siblings ...)
2008-01-11 4:51 ` raksit at gcc dot gnu dot org
@ 2008-01-14 19:57 ` raksit at gcc dot gnu dot org
5 siblings, 0 replies; 7+ messages in thread
From: raksit at gcc dot gnu dot org @ 2008-01-14 19:57 UTC (permalink / raw)
To: gcc-bugs
------- Comment #6 from raksit at gcc dot gnu dot org 2008-01-14 19:27 -------
This was fixed in revision 131460.
--
raksit at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution| |FIXED
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2008-01-14 19:27 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2006-06-09 6:18 [Bug pending/27971] New: eliminate shift in array[(x>>2)&3] dean at arctic dot org
2006-06-09 6:20 ` [Bug target/27971] " pinskia at gcc dot gnu dot org
2006-06-09 6:43 ` pinskia at gcc dot gnu dot org
2007-11-28 23:06 ` raksit at google dot com
2007-12-05 19:28 ` raksit at google dot com
2008-01-11 4:51 ` raksit at gcc dot gnu dot org
2008-01-14 19:57 ` raksit at gcc dot gnu dot org
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