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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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