Re: powerpc: Discard ffs() function and use builtin_ffs instead
Le 13/05/2016 à 08:53, Christophe Leroy a écrit : Le 13/05/2016 à 08:16, Michael Ellerman a écrit : On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: With the ffs() function as defined in arch/powerpc/include/asm/bitops.h GCC will not optimise the code in case of constant parameter, as shown by the small exemple below. int ffs_test(void) { return 4 << ffs(31); } c0012334 : c0012334: 39 20 00 01 li r9,1 c0012338: 38 60 00 04 li r3,4 c001233c: 7d 29 00 34 cntlzw r9,r9 c0012340: 21 29 00 20 subfic r9,r9,32 c0012344: 7c 63 48 30 slw r3,r3,r9 c0012348: 4e 80 00 20 blr With this patch, the same function will compile as follows: c0012334 : c0012334: 38 60 00 08 li r3,8 c0012338: 4e 80 00 20 blr But what code does it generate when it's not a constant? The generated code is the same with and without the patch when not a constant: int ffs_test2(int x) { return ffs(x); } c001233c : c001233c: 7d 23 00 d0 neg r9,r3 c0012340: 7d 23 18 38 and r3,r9,r3 c0012344: 7c 63 00 34 cntlzw r3,r3 c0012348: 20 63 00 20 subfic r3,r3,32 c001234c: 4e 80 00 20 blr And which gcc version first added the builtin version? Don't know, but __builtin_ffs() is already used in arch/powerpc/include/asm/page_32.h Hi Michael, Any change to get it into 4.9 ? Christophe
Re: powerpc: Discard ffs() function and use builtin_ffs instead
Le 13/05/2016 à 08:53, Christophe Leroy a écrit : Le 13/05/2016 à 08:16, Michael Ellerman a écrit : On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: With the ffs() function as defined in arch/powerpc/include/asm/bitops.h GCC will not optimise the code in case of constant parameter, as shown by the small exemple below. int ffs_test(void) { return 4 << ffs(31); } c0012334 : c0012334: 39 20 00 01 li r9,1 c0012338: 38 60 00 04 li r3,4 c001233c: 7d 29 00 34 cntlzw r9,r9 c0012340: 21 29 00 20 subfic r9,r9,32 c0012344: 7c 63 48 30 slw r3,r3,r9 c0012348: 4e 80 00 20 blr With this patch, the same function will compile as follows: c0012334 : c0012334: 38 60 00 08 li r3,8 c0012338: 4e 80 00 20 blr But what code does it generate when it's not a constant? The generated code is the same with and without the patch when not a constant: int ffs_test2(int x) { return ffs(x); } c001233c : c001233c: 7d 23 00 d0 neg r9,r3 c0012340: 7d 23 18 38 and r3,r9,r3 c0012344: 7c 63 00 34 cntlzw r3,r3 c0012348: 20 63 00 20 subfic r3,r3,32 c001234c: 4e 80 00 20 blr And which gcc version first added the builtin version? Don't know, but __builtin_ffs() is already used in arch/powerpc/include/asm/page_32.h Hi Michael, Any change to get it into 4.9 ? Christophe
Re: powerpc: Discard ffs() function and use builtin_ffs instead
On Fri, May 13, 2016 at 04:16:57PM +1000, Michael Ellerman wrote: > On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: > > With the ffs() function as defined in arch/powerpc/include/asm/bitops.h > > GCC will not optimise the code in case of constant parameter, as shown > > by the small exemple below. > > > > int ffs_test(void) > > { > > return 4 << ffs(31); > > } > > > > c0012334 : > > c0012334: 39 20 00 01 li r9,1 > > c0012338: 38 60 00 04 li r3,4 > > c001233c: 7d 29 00 34 cntlzw r9,r9 > > c0012340: 21 29 00 20 subfic r9,r9,32 > > c0012344: 7c 63 48 30 slw r3,r3,r9 > > c0012348: 4e 80 00 20 blr > > > > With this patch, the same function will compile as follows: > > > > c0012334 : > > c0012334: 38 60 00 08 li r3,8 > > c0012338: 4e 80 00 20 blr > > > But what code does it generate when it's not a constant? > > And which gcc version first added the builtin version? It already existed in gcc-2.95, which you do not want to use to compile anything today but I have in a corner of a chroot environment to maintain ~1997 vintage embedded stuff, running a 2.2.12 kernel! Hopefully this clears up your concerns :-) Cheers, Gabriel
Re: powerpc: Discard ffs() function and use builtin_ffs instead
On Fri, May 13, 2016 at 04:16:57PM +1000, Michael Ellerman wrote: > On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: > > With the ffs() function as defined in arch/powerpc/include/asm/bitops.h > > GCC will not optimise the code in case of constant parameter, as shown > > by the small exemple below. > > > > int ffs_test(void) > > { > > return 4 << ffs(31); > > } > > > > c0012334 : > > c0012334: 39 20 00 01 li r9,1 > > c0012338: 38 60 00 04 li r3,4 > > c001233c: 7d 29 00 34 cntlzw r9,r9 > > c0012340: 21 29 00 20 subfic r9,r9,32 > > c0012344: 7c 63 48 30 slw r3,r3,r9 > > c0012348: 4e 80 00 20 blr > > > > With this patch, the same function will compile as follows: > > > > c0012334 : > > c0012334: 38 60 00 08 li r3,8 > > c0012338: 4e 80 00 20 blr > > > But what code does it generate when it's not a constant? > > And which gcc version first added the builtin version? It already existed in gcc-2.95, which you do not want to use to compile anything today but I have in a corner of a chroot environment to maintain ~1997 vintage embedded stuff, running a 2.2.12 kernel! Hopefully this clears up your concerns :-) Cheers, Gabriel
Re: powerpc: Discard ffs() function and use builtin_ffs instead
Le 13/05/2016 à 08:16, Michael Ellerman a écrit : On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: With the ffs() function as defined in arch/powerpc/include/asm/bitops.h GCC will not optimise the code in case of constant parameter, as shown by the small exemple below. int ffs_test(void) { return 4 << ffs(31); } c0012334 : c0012334: 39 20 00 01 li r9,1 c0012338: 38 60 00 04 li r3,4 c001233c: 7d 29 00 34 cntlzw r9,r9 c0012340: 21 29 00 20 subfic r9,r9,32 c0012344: 7c 63 48 30 slw r3,r3,r9 c0012348: 4e 80 00 20 blr With this patch, the same function will compile as follows: c0012334 : c0012334: 38 60 00 08 li r3,8 c0012338: 4e 80 00 20 blr But what code does it generate when it's not a constant? The generated code is the same with and without the patch when not a constant: int ffs_test2(int x) { return ffs(x); } c001233c : c001233c: 7d 23 00 d0 neg r9,r3 c0012340: 7d 23 18 38 and r3,r9,r3 c0012344: 7c 63 00 34 cntlzw r3,r3 c0012348: 20 63 00 20 subfic r3,r3,32 c001234c: 4e 80 00 20 blr And which gcc version first added the builtin version? Don't know, but __builtin_ffs() is already used in arch/powerpc/include/asm/page_32.h Christophe
Re: powerpc: Discard ffs() function and use builtin_ffs instead
Le 13/05/2016 à 08:16, Michael Ellerman a écrit : On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: With the ffs() function as defined in arch/powerpc/include/asm/bitops.h GCC will not optimise the code in case of constant parameter, as shown by the small exemple below. int ffs_test(void) { return 4 << ffs(31); } c0012334 : c0012334: 39 20 00 01 li r9,1 c0012338: 38 60 00 04 li r3,4 c001233c: 7d 29 00 34 cntlzw r9,r9 c0012340: 21 29 00 20 subfic r9,r9,32 c0012344: 7c 63 48 30 slw r3,r3,r9 c0012348: 4e 80 00 20 blr With this patch, the same function will compile as follows: c0012334 : c0012334: 38 60 00 08 li r3,8 c0012338: 4e 80 00 20 blr But what code does it generate when it's not a constant? The generated code is the same with and without the patch when not a constant: int ffs_test2(int x) { return ffs(x); } c001233c : c001233c: 7d 23 00 d0 neg r9,r3 c0012340: 7d 23 18 38 and r3,r9,r3 c0012344: 7c 63 00 34 cntlzw r3,r3 c0012348: 20 63 00 20 subfic r3,r3,32 c001234c: 4e 80 00 20 blr And which gcc version first added the builtin version? Don't know, but __builtin_ffs() is already used in arch/powerpc/include/asm/page_32.h Christophe
Re: powerpc: Discard ffs() function and use builtin_ffs instead
On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: > With the ffs() function as defined in arch/powerpc/include/asm/bitops.h > GCC will not optimise the code in case of constant parameter, as shown > by the small exemple below. > > int ffs_test(void) > { > return 4 << ffs(31); > } > > c0012334 : > c0012334: 39 20 00 01 li r9,1 > c0012338: 38 60 00 04 li r3,4 > c001233c: 7d 29 00 34 cntlzw r9,r9 > c0012340: 21 29 00 20 subfic r9,r9,32 > c0012344: 7c 63 48 30 slw r3,r3,r9 > c0012348: 4e 80 00 20 blr > > With this patch, the same function will compile as follows: > > c0012334 : > c0012334: 38 60 00 08 li r3,8 > c0012338: 4e 80 00 20 blr But what code does it generate when it's not a constant? And which gcc version first added the builtin version? cheers
Re: powerpc: Discard ffs() function and use builtin_ffs instead
On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote: > With the ffs() function as defined in arch/powerpc/include/asm/bitops.h > GCC will not optimise the code in case of constant parameter, as shown > by the small exemple below. > > int ffs_test(void) > { > return 4 << ffs(31); > } > > c0012334 : > c0012334: 39 20 00 01 li r9,1 > c0012338: 38 60 00 04 li r3,4 > c001233c: 7d 29 00 34 cntlzw r9,r9 > c0012340: 21 29 00 20 subfic r9,r9,32 > c0012344: 7c 63 48 30 slw r3,r3,r9 > c0012348: 4e 80 00 20 blr > > With this patch, the same function will compile as follows: > > c0012334 : > c0012334: 38 60 00 08 li r3,8 > c0012338: 4e 80 00 20 blr But what code does it generate when it's not a constant? And which gcc version first added the builtin version? cheers