[libc][gpu] Disable loop unrolling in the throughput benchmark loop (#153971)
This patch makes GPU throughput benchmark results more comparable across
targets by disabling loop unrolling in the benchmark loop.
Motivation:
* PTX (post-LTO) evidence on NVPTX: for libc `sin`, the generated PTX
shows the `throughput` loop unrolled 8x at `N=128` (one iteration
advances the input pointer by 64 bytes = 8 doubles), interleaving eight
independent chains before the back-edge. This hides latency and
significantly reduces cycles/call as the batch size `N` grows.
* Observed scaling (NVPTX measurements): with unrolling enabled, `sin`
dropped from ~3,100 cycles/call at `N=1` to ~360 at `N=128`. After
enforcing `#pragma clang loop unroll(disable)`, results stabilized
(e.g., from ~3100 cycles/call at `N=1` to ~2700 at `N=128`).
* libdevice contrast: the libdevice `sin` path did not exhibit a similar
drop in our measurements, and the PTX appears as compact internal calls
rather than a long FMA chain, leaving less ILP for the outer loop to
extract.
What this change does:
* Applies `#pragma clang loop unroll(disable)` to the GPU `throughput()`
loop in both NVPTX and AMDGPU backends.
Leaving unrolling entirely to the optimizer makes apples-to-apples
comparisons uneven (e.g., libc vs. vendor). Disabling unrolling yields
fairer, more consistent numbers.
GitOrigin-RevId: 75bf7392089d027bb6fa78ded21acaa97b16a412
2 files changed
