Hi!
I've attempted to study the implementation of memcpy for 32-bit Arm cores in
Glibc (which is also found in arm-optimized-routines and first appeared in
Linaro's cortex-strings project), and I came across a peculiar snippet:
#ifdef USE_VFP
/* Magic dust alert! Force VFP on Cortex-A9. Experiments show
that the FP pipeline is much better at streaming loads and
stores. This is outside the critical loop. */
vmov.f32 s0, s0
#endif
This seems to imply that this NOP-like instruction affects CPU state and makes
the vldr/vstr instructions that follow use different datapaths that they might
otherwise? Can anyone shed more light on this, please?
I was able to trace history of this code back to revision 100 in cortex-strings
repository, where it appeared as part of a large rewrite by Will Newton:
https://bazaar.launchpad.net/~linaro-toolchain-dev/cortex-strings/trunk/revision/100/src/linaro-a9/memcpy.S
The entire memcpy.S file in Arm optimized-routines repo can be found here:
https://github.com/ARM-software/optimized-routines/blob/master/string/arm/memcpy.S
Thanks!
Alexander
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