llvm-project

75bf7392 - [libc][gpu] Disable loop unrolling in the throughput benchmark loop (#153971)

[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.