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From: "wvangulik at xs4all dot nl" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/33970] Missed optimization using unsigned char loop variable Date: Tue, 06 Nov 2007 19:35:00 -0000 [thread overview] Message-ID: <20071106193459.613.qmail@sourceware.org> (raw) In-Reply-To: <bug-33970-15294@http.gcc.gnu.org/bugzilla/> ------- Comment #10 from wvangulik at xs4all dot nl 2007-11-06 19:34 ------- (In reply to comment #9) > > I think you will also find that if x is changed from ststic to auto the same > problem appears. > Ok, I tried to find the minimum test case. And it has nothing todo with static/volatile/inline etc. ============================================== int sub2(unsigned char); // external function void foo(void) { unsigned char x; for(x=0;x<128; x++) { //sub2(x); //x is becomes a int (16bit) sub2(x+1); //x is char (8bit) } } All is compiled using 4.1.2 and "avr-gcc -Wall -Os -mmcu=avr5 -S test.c" The output when compiling with "sub2(x)" ================================================= /* prologue: frame size=0 */ push r28 push r29 /* prologue end (size=2) */ ldi r28,lo8(0) ldi r29,hi8(0) .L2: mov r24,r28 ; << loads as a byte! call sub2 adiw r28,1 ; << increment as a int cpi r28,128 ; << compare as a int cpc r29,__zero_reg__ brne .L2 /* epilogue: frame size=0 */ pop r29 pop r28 ret The output when compiling with "sub2(x+1)" ================================================================ /* prologue: frame size=0 */ push r17 /* prologue end (size=1) */ ldi r17,lo8(0) .L2: subi r17,lo8(-(1)) mov r24,r17 call sub2 cpi r17,lo8(-128) brne .L2 /* epilogue: frame size=0 */ pop r17 ret =========================================================== >From compiling with x as a int I got a sort of hint. When using x+1, x is actually loaded with 1 (and not zero). So i tried: ====================================================== void foo(void) { unsigned char x; for(x=1;x<129; x++) sub2(x); } And it gives the assembler loop check as for the "sub(x+1)" variant of course the loop is now effectivley the same as "sub2(x+1)". But the the incrementing of x is now done after the call, so it proofs that this is not the problem. I also tried different loop lengths, thinking 128 is the nasty one, but it did not help. ====================================================== /* prologue: frame size=0 */ push r17 /* prologue end (size=1) */ ldi r17,lo8(1) .L2: mov r24,r17 call sub2 subi r17,lo8(-(1)) cpi r17,lo8(-127) brne .L2 /* epilogue: frame size=0 */ pop r17 ret I am out of knowledge now, I do not know how to further debug GCC on this. Just hoping this will help someone to pinpoint the problem. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33970
next prev parent reply other threads:[~2007-11-06 19:35 UTC|newest] Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top 2007-11-01 13:44 [Bug c/33970] New: " henning dot m at insightbb dot com 2007-11-01 14:10 ` [Bug c/33970] " rguenth at gcc dot gnu dot org 2007-11-01 17:09 ` [Bug middle-end/33970] " pinskia at gcc dot gnu dot org 2007-11-01 17:28 ` eweddington at cso dot atmel dot com 2007-11-01 17:45 ` eweddington at cso dot atmel dot com 2007-11-01 17:47 ` eweddington at cso dot atmel dot com 2007-11-01 21:26 ` henning dot m at insightbb dot com 2007-11-04 23:28 ` eweddington at cso dot atmel dot com 2007-11-04 23:28 ` eweddington at cso dot atmel dot com 2007-11-05 22:48 ` wvangulik at xs4all dot nl 2007-11-06 12:37 ` henning dot m at insightbb dot com 2007-11-06 19:35 ` wvangulik at xs4all dot nl [this message] 2007-11-06 21:01 ` wvangulik at xs4all dot nl 2010-09-13 12:09 ` abnikant dot singh at atmel dot com [not found] <bug-33970-4@http.gcc.gnu.org/bugzilla/> 2011-07-20 17:48 ` gjl at gcc dot gnu.org
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