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From: "semicontinuity at yandex dot ru" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/58405] New: Unoptimal code generated for computed goto Date: Thu, 12 Sep 2013 11:15:00 -0000 [thread overview] Message-ID: <bug-58405-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58405 Bug ID: 58405 Summary: Unoptimal code generated for computed goto Product: gcc Version: 4.7.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: semicontinuity at yandex dot ru The following code // ----------------------------------------------------------------------------- static void test(void) { char flag = 1; static void* address = &&L0; goto *address; for(;;) { L0: asm volatile("nop\t\n"); flag = 0; L1: (void)&&L1; if (flag==0) { address = &&L1; return; } asm volatile("nop\t\n"); flag = 0; L2: (void)&&L2; if (flag==0) { address = &&L2; return; } } } int main(void) { for(;;) { test(); } return 0; } // ----------------------------------------------------------------------------- Compiles to: // ----------------------------------------------------------------------------- 00000052 <test>: static void test(void) { 52: cf 93 push r28 54: df 93 push r29 56: 00 d0 rcall .+0 ; 0x58 <test+0x6> 58: cd b7 in r28, 0x3d ; 61 5a: de b7 in r29, 0x3e ; 62 5c: 80 91 60 00 lds r24, 0x0060 60: 90 91 61 00 lds r25, 0x0061 64: 8f 93 push r24 66: 9f 93 push r25 68: 08 95 ret goto *address; for(;;) { L0: asm volatile("nop\t\n"); 6a: 00 00 nop flag = 0; L1: (void)&&L1; if (flag==0) { address = &&L1; return; } 6c: 81 e4 ldi r24, 0x41 ; 65 6e: 90 e0 ldi r25, 0x00 ; 0 70: 90 93 61 00 sts 0x0061, r25 74: 80 93 60 00 sts 0x0060, r24 flag = 0; L2: (void)&&L2; if (flag==0) { address = &&L2; return; } } } 78: 0f 90 pop r0 7a: 0f 90 pop r0 7c: df 91 pop r29 7e: cf 91 pop r28 80: 08 95 ret ... // ----------------------------------------------------------------------------- This code works but is non optimal: - It saves stack pointer to r28:r29, but later r28:r29 is never used - Because it uses r28:r29, it saves r28:r29 to stack. - It makes rcall .+0 and later removes 2 bytes from stack (pop r0) - this is not necessary; rcall .+0 and pop r0's can be removed. - Computed goto is implemented with 5 instructions, 7 words, with stack usage: 5c: 80 91 60 00 lds r24, 0x0060 60: 90 91 61 00 lds r25, 0x0061 64: 8f 93 push r24 66: 9f 93 push r25 68: 08 95 ret Instead, it could be implemented with lds r30, 0x0060 lds r31, 0x0061 ijmp with 3 instructions, 5 words, no additional memory access, but with clobbering of r30:r31 instead of r24:r25 which is OK. // ------------------------------------ Compiling C: main.c avr-gcc -c -mmcu=atmega8535 -I. -gdwarf-2 -I../../bsp -I../../../.. -O3 -save-temps -v -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -Wundef -Wa,-adhlns=.obj/main.lst -std=gnu99 -Wundef -MD -MP -MF .dep/main.o.d main.c -o .obj/main.o Using built-in specs. COLLECT_GCC=avr-gcc Target: avr Configured with: ../../gcc.gnu.org/gcc-4_7-branch/configure --target=avr --prefix=/local/gnu/install/gcc-4.7-mingw32 --host=i386-mingw32 --build=i686-linux-gnu --enable-languages=c,c++ --disable-nls --disable-shared --with-dwarf2 --with-avrlibc=yes Thread model: single gcc version 4.7.2 (GCC)
next reply other threads:[~2013-09-12 11:15 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-09-12 11:15 semicontinuity at yandex dot ru [this message] 2013-09-14 18:30 ` [Bug target/58405] " gjl at gcc dot gnu.org 2013-09-14 18:30 ` gjl at gcc dot gnu.org 2013-09-16 16:14 ` gjl at gcc dot gnu.org
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