On 2/14/24 22:57, Alexander Monakov wrote:
On Wed, 14 Feb 2024, Richard Henderson wrote:
v3: https://patchew.org/QEMU/20240206204809.9859-1-amona...@ispras.ru/
Changes for v4:
- Keep separate >= 256 entry point, but only keep constant length
check inline. This allows the indirect function call to be hidden
and optimized away when the pointer is constant.
Sorry, I don't understand this. Most of the improvement (at least in our
testing) comes from inlining the byte checks, which often fail and eliminate
call overhead entirely. Moving them out-of-line seems to lose most of the
speedup the patchset was bringing, doesn't it? Is there some concern I am
not seeing?
What is your benchmarking method?
It was my guess that most of the improvement came from performing those early byte checks
*at all*, and that the overhead of a function call to a small out of line wrapper would be
negligible.
By not exposing the function pointer outside the bufferiszero translation unit, the
compiler can see when the pointer is never modified for a given host, and then transform
the indirect branch to a direct branch.
r~
