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* [Bug tree-optimization/31248] New: Too much casting for char operands on tree level @ 2007-03-17 19:09 ubizjak at gmail dot com 2007-03-18 5:30 ` [Bug tree-optimization/31248] " pinskia at gcc dot gnu dot org ` (4 more replies) 0 siblings, 5 replies; 6+ messages in thread From: ubizjak at gmail dot com @ 2007-03-17 19:09 UTC (permalink / raw) To: gcc-bugs Following testcase produces unnecessary moves in short loop: --cut here-- char table[256]; int test(void) { char val = 0; int i; for (i = 0; i < 10; i++) val += table[i]; return val; } --cut here-- gcc -O2: test: .LFB2: movzbl table(%rip), %ecx movl $1, %edx .p2align 4,,7 .L2: (*) movl %ecx, %eax addb table(%rdx), %al addq $1, %rdx cmpq $10, %rdx (*) movl %eax, %ecx jne .L2 movsbl %al,%eax ret Those unneccesary moves (*) are generated due to many (spurious?) casts in optimized tree dump: test () { long unsigned int ivtmp.31; char val; unsigned char D.1985; <bb 2>: val = (char) (unsigned char) MEM[symbol: table]; ivtmp.31 = 1; <L0>:; D.1985 = (unsigned char) val + (unsigned char) MEM[symbol: table, index: ivtmp.31]; val = (char) D.1985; ivtmp.31 = ivtmp.31 + 1; if (ivtmp.31 != 10) goto <L0>; else goto <L2>; <L2>:; return (int) val; -- Summary: Too much casting for char operands on tree level Product: gcc Version: 4.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: ubizjak at gmail dot com GCC build triplet: x86_64-pc-linux-gnu GCC host triplet: x86_64-pc-linux-gnu GCC target triplet: x86_64-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248 ^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/31248] Too much casting for char operands on tree level 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com @ 2007-03-18 5:30 ` pinskia at gcc dot gnu dot org 2007-03-29 6:16 ` [Bug rtl-optimization/31248] char adding gives an extra move or two pinskia at gcc dot gnu dot org ` (3 subsequent siblings) 4 siblings, 0 replies; 6+ messages in thread From: pinskia at gcc dot gnu dot org @ 2007-03-18 5:30 UTC (permalink / raw) To: gcc-bugs ------- Comment #1 from pinskia at gcc dot gnu dot org 2007-03-18 05:29 ------- You are wrong. The casts are correct as char + char in C is actually defined as (int)(char) + (int) (char) aka promoted to int while doing the addition so what the optimizers do when it sees (char)((int)(char) + (int)(char)), it changes the addition to be (unsigned char)(char) + (unsigned char)(char) so the addition's overflow is defined as being wrapping instead of being undefined. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248 ^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug rtl-optimization/31248] char adding gives an extra move or two 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com 2007-03-18 5:30 ` [Bug tree-optimization/31248] " pinskia at gcc dot gnu dot org @ 2007-03-29 6:16 ` pinskia at gcc dot gnu dot org 2007-08-28 12:02 ` [Bug rtl-optimization/31248] char adding (in loops) " ubizjak at gmail dot com ` (2 subsequent siblings) 4 siblings, 0 replies; 6+ messages in thread From: pinskia at gcc dot gnu dot org @ 2007-03-29 6:16 UTC (permalink / raw) To: gcc-bugs ------- Comment #2 from pinskia at gcc dot gnu dot org 2007-03-29 07:15 ------- I am going to mark this as a rtl optimization issue because the tree level is correct unless we get a PLUS_EXPR which is able to wrap without being undefined. -- pinskia at gcc dot gnu dot org changed: What |Removed |Added ---------------------------------------------------------------------------- Severity|normal |enhancement Component|tree-optimization |rtl-optimization Summary|Too much casting for char |char adding gives an extra |operands on tree level |move or two http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248 ^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug rtl-optimization/31248] char adding (in loops) gives an extra move or two 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com 2007-03-18 5:30 ` [Bug tree-optimization/31248] " pinskia at gcc dot gnu dot org 2007-03-29 6:16 ` [Bug rtl-optimization/31248] char adding gives an extra move or two pinskia at gcc dot gnu dot org @ 2007-08-28 12:02 ` ubizjak at gmail dot com 2008-01-09 9:29 ` ubizjak at gmail dot com 2008-01-09 9:46 ` ubizjak at gmail dot com 4 siblings, 0 replies; 6+ messages in thread From: ubizjak at gmail dot com @ 2007-08-28 12:02 UTC (permalink / raw) To: gcc-bugs ------- Comment #3 from ubizjak at gmail dot com 2007-08-28 12:02 ------- Current mainline [GCC: (GNU) 4.3.0 20070828] generates: test: .LFB2: xorl %eax, %eax xorl %edx, %edx .align 16 .L2: addb table(%rdx), %al addq $1, %rdx cmpq $10, %rdx jne .L2 movsbl %al,%eax ret Now, there is an optimization problem in the intialization code in loop header. To omit one iteration, it should start with: .LFB2: movzbl table(%rip), %ecx movl $1, %edx .L2: ... BTW: By reversing the loop: for (i = 9; i; i--) val += table[i]; we could remove comparison from the loop, but instead we produce (x86_64): test: .LFB2: xorl %eax, %eax xorl %edx, %edx .align 16 .L2: addb table+9(%rdx), %al subq $1, %rdx cmpq $-9, %rdx jne .L2 movsbl %al,%eax ret However, using -m32 we get: test: xorl %eax, %eax movl $9, %edx .align 16 .L2: addb table(%edx), %al subl $1, %edx jne .L2 movsbl %al,%eax ret In reversed-loop case, we could generate: test: movl $9, %edx movzbl table(%edx), %eax .align 16 .L2: addb table(%edx), %al subl $1, %edx jne .L2 movsbl %al,%eax ret -- ubizjak at gmail dot com changed: What |Removed |Added ---------------------------------------------------------------------------- Summary|char adding gives an extra |char adding (in loops) gives |move or two |an extra move or two http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248 ^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug rtl-optimization/31248] char adding (in loops) gives an extra move or two 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com ` (2 preceding siblings ...) 2007-08-28 12:02 ` [Bug rtl-optimization/31248] char adding (in loops) " ubizjak at gmail dot com @ 2008-01-09 9:29 ` ubizjak at gmail dot com 2008-01-09 9:46 ` ubizjak at gmail dot com 4 siblings, 0 replies; 6+ messages in thread From: ubizjak at gmail dot com @ 2008-01-09 9:29 UTC (permalink / raw) To: gcc-bugs ------- Comment #4 from ubizjak at gmail dot com 2008-01-09 08:23 ------- The problem with char moves is fixed in current SVN. The problem with loop header from comment #3 is tree-optimization problem, I'll open new PR for it. -- ubizjak at gmail dot com changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution| |FIXED http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248 ^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug rtl-optimization/31248] char adding (in loops) gives an extra move or two 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com ` (3 preceding siblings ...) 2008-01-09 9:29 ` ubizjak at gmail dot com @ 2008-01-09 9:46 ` ubizjak at gmail dot com 4 siblings, 0 replies; 6+ messages in thread From: ubizjak at gmail dot com @ 2008-01-09 9:46 UTC (permalink / raw) To: gcc-bugs ------- Comment #5 from ubizjak at gmail dot com 2008-01-09 08:33 ------- -> PR tree-optimization/34723 -- ubizjak at gmail dot com changed: What |Removed |Added ---------------------------------------------------------------------------- Target Milestone|--- |4.3.0 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248 ^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2008-01-09 8:34 UTC | newest] Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com 2007-03-18 5:30 ` [Bug tree-optimization/31248] " pinskia at gcc dot gnu dot org 2007-03-29 6:16 ` [Bug rtl-optimization/31248] char adding gives an extra move or two pinskia at gcc dot gnu dot org 2007-08-28 12:02 ` [Bug rtl-optimization/31248] char adding (in loops) " ubizjak at gmail dot com 2008-01-09 9:29 ` ubizjak at gmail dot com 2008-01-09 9:46 ` ubizjak at gmail dot com
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