patch for hp-timing.h

[Tang, Jun]

Wilco,

Removing zero bits from multiply and making shifts afterwards can extend the time before overflow.  Does it also reduce the resolution of the timer?

Regards,

Jun

-----Original Message-----
From: Wilco Dijkstra <Wilco.Dijkstra@arm.com> 
Sent: Friday, February 3, 2023 7:30 AM
To: Tang, Jun <juntangc@amazon.com>
Cc: 'GNU C Library' <libc-alpha@sourceware.org>
Subject: [EXTERNAL] patch for hp-timing.h

CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.



Hi Jun,

> hp-timing is only used on benchtest today, but it can be used on other 
> measurements in the future.

It can't as proposed below since this would overflow in ~16 seconds.
We could improve this by removing the zero bits from the multiply and doing some shifts, maybe that will make the interval long enough for integer multiply. Otherwise I'd suggest the floating point version since it's not like the benchtests don't already use floating point.

Cheers,
Wilco

diff --git a/benchtests/Makefile b/benchtests/Makefile index 292976b26b..a624614207 100644
--- a/benchtests/Makefile
+++ b/benchtests/Makefile
@@ -499,4 +499,5 @@ $(objpfx)bench-%.c: %-inputs $(bench-deps)
          cat $($*-INCLUDE); \
        fi; \
        $(PYTHON) scripts/bench.py $(patsubst %-inputs,%,$<); } > $@-tmp
+       cp -f $@-tmp $@-bak

Unintended change I guess?

        mv -f $@-tmp $@
diff --git a/sysdeps/aarch64/hp-timing.h b/sysdeps/aarch64/hp-timing.h index f7f7ac7cae..c699effe6a 100644
--- a/sysdeps/aarch64/hp-timing.h
+++ b/sysdeps/aarch64/hp-timing.h
@@ -41,7 +41,7 @@ typedef uint64_t hp_timing_t;
 #define HP_TIMING_DIFF(Diff, Start, End)                       \
 ({  hp_timing_t freq;                                          \
     __asm__ __volatile__ ("mrs %0, cntfrq_el0" : "=r" (freq)); \
-   (Diff) = ((End) - (Start)) * (UINT64_C(1000000000) / freq); \
+   (Diff) = (((End) - (Start)) * UINT64_C(1000000000)) / freq; \
 })

 #endif /* hp-timing.h */