| Message ID | 1394080919-17957-4-git-send-email-sukadev@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
From: Sukadev Bhattiprolu > When checking whether a bit representing a register is set in > sample_regs, a 64-bit mask, use 64-bit value (1LL). > > Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> > --- > tools/perf/util/unwind.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c > index 742f23b..2b888c6 100644 > --- a/tools/perf/util/unwind.c > +++ b/tools/perf/util/unwind.c > @@ -396,11 +396,11 @@ static int reg_value(unw_word_t *valp, struct regs_dump *regs, int id, > { > int i, idx = 0; > > - if (!(sample_regs & (1 << id))) > + if (!(sample_regs & (1LL << id))) > return -EINVAL; > > for (i = 0; i < id; i++) { > - if (sample_regs & (1 << i)) > + if (sample_regs & (1LL << i)) > idx++; > } There are much faster ways to count the number of set bits, especially if you might need to check a significant number of bits. There might even be a function defined somewhere to do it. Basically you just add up the bits, for 16 bit it would be: val = (val & 0x5555) + (val >> 1) & 0x5555; val = (val & 0x3333) + (val >> 2) & 0x3333; val = (val & 0x0f0f) + (val >> 4) & 0x0f0f; val = (val & 0x00ff) + (val >> 8) & 0x00ff; As the size of the work increases the improvement is more significant. (Some of the later masking can probably be proven unnecessary.) David
On Thu, Mar 06, 2014 at 09:44:47AM +0000, David Laight wrote: > From: Sukadev Bhattiprolu > > When checking whether a bit representing a register is set in > > sample_regs, a 64-bit mask, use 64-bit value (1LL). > > > > Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> > > --- > > tools/perf/util/unwind.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c > > index 742f23b..2b888c6 100644 > > --- a/tools/perf/util/unwind.c > > +++ b/tools/perf/util/unwind.c > > @@ -396,11 +396,11 @@ static int reg_value(unw_word_t *valp, struct regs_dump *regs, int id, > > { > > int i, idx = 0; > > > > - if (!(sample_regs & (1 << id))) > > + if (!(sample_regs & (1LL << id))) > > return -EINVAL; > > > > for (i = 0; i < id; i++) { > > - if (sample_regs & (1 << i)) > > + if (sample_regs & (1LL << i)) > > idx++; > > } > > There are much faster ways to count the number of set bits, especially > if you might need to check a significant number of bits. > There might even be a function defined somewhere to do it. Indeed, look for Hamming weight (hweight family of functions) in asm/hweight.h and what is included from there. Besides that, many modern processors also have a machine instruction to perform this task. In the processor manuals the instruction is described as population count and the mnemonic starts with "popcnt" on x86 and ppc. Gabriel > Basically you just add up the bits, for 16 bit it would be: > val = (val & 0x5555) + (val >> 1) & 0x5555; > val = (val & 0x3333) + (val >> 2) & 0x3333; > val = (val & 0x0f0f) + (val >> 4) & 0x0f0f; > val = (val & 0x00ff) + (val >> 8) & 0x00ff; > As the size of the work increases the improvement is more significant. > (Some of the later masking can probably be proven unnecessary.) > > David > > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev
On Thu, Mar 06, 2014 at 12:33:32PM +0100, Gabriel Paubert wrote: > On Thu, Mar 06, 2014 at 09:44:47AM +0000, David Laight wrote: > > From: Sukadev Bhattiprolu > > > When checking whether a bit representing a register is set in > > > sample_regs, a 64-bit mask, use 64-bit value (1LL). > > > > > > Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> > > > --- > > > tools/perf/util/unwind.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c > > > index 742f23b..2b888c6 100644 > > > --- a/tools/perf/util/unwind.c > > > +++ b/tools/perf/util/unwind.c > > > @@ -396,11 +396,11 @@ static int reg_value(unw_word_t *valp, struct regs_dump *regs, int id, > > > { > > > int i, idx = 0; > > > > > > - if (!(sample_regs & (1 << id))) > > > + if (!(sample_regs & (1LL << id))) > > > return -EINVAL; > > > > > > for (i = 0; i < id; i++) { > > > - if (sample_regs & (1 << i)) > > > + if (sample_regs & (1LL << i)) > > > idx++; > > > } > > > > There are much faster ways to count the number of set bits, especially > > if you might need to check a significant number of bits. > > There might even be a function defined somewhere to do it. > > Indeed, look for Hamming weight (hweight family of functions) > in asm/hweight.h and what is included from there. > > Besides that, many modern processors also have a machine instruction > to perform this task. In the processor manuals the instruction is > described as population count and the mnemonic starts with "popcnt" > on x86 and ppc. > > Gabriel > > > Basically you just add up the bits, for 16 bit it would be: > > val = (val & 0x5555) + (val >> 1) & 0x5555; > > val = (val & 0x3333) + (val >> 2) & 0x3333; > > val = (val & 0x0f0f) + (val >> 4) & 0x0f0f; > > val = (val & 0x00ff) + (val >> 8) & 0x00ff; > > As the size of the work increases the improvement is more significant. > > (Some of the later masking can probably be proven unnecessary.) right I think the loop could be replaced by: idx = hweight(mask & ((1 << id) - 1)) Sukadev, please also rebase against latest Arnaldo's perf/core, this code has changed just recently, it's now in: util/perf_regs.c:perf_reg_value thanks, jirka
diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c index 742f23b..2b888c6 100644 --- a/tools/perf/util/unwind.c +++ b/tools/perf/util/unwind.c @@ -396,11 +396,11 @@ static int reg_value(unw_word_t *valp, struct regs_dump *regs, int id, { int i, idx = 0; - if (!(sample_regs & (1 << id))) + if (!(sample_regs & (1LL << id))) return -EINVAL; for (i = 0; i < id; i++) { - if (sample_regs & (1 << i)) + if (sample_regs & (1LL << i)) idx++; }
When checking whether a bit representing a register is set in sample_regs, a 64-bit mask, use 64-bit value (1LL). Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> --- tools/perf/util/unwind.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)