[clang] [llvm] [offload][OpenMP] Improve cross-team reduction grid selection (PR #208253)

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@llvm/pr-subscribers-offload

Author: Robert Imschweiler (ro-i)

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<summary>Changes</summary>

The default cross-team reduction algorithm benefits from larger and fewer teams. Implement that by using 2 x the default number of threads and 1/2 x the default number of teams for reduction kernels.  This doesn't change the default total number of threads, it just redistributes them.

This is a first, rather simple heuristic, derived from (a subset of) what AOMP does.
The performance benefits I observed for the reduction tests in https://github.com/ro-i/xteam-test on a gfx942
(c71339705091500f731e2a39f247d2660bacbdce) are up to a few percent, with no regressions.

Claude assisted with this patch.

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Full diff: https://github.com/llvm/llvm-project/pull/208253.diff


3 Files Affected:

- (modified) clang/lib/CodeGen/CGOpenMPRuntimeGPU.cpp (+22) 
- (modified) offload/plugins-nextgen/common/include/PluginInterface.h (+5) 
- (modified) offload/plugins-nextgen/common/src/PluginInterface.cpp (+22-4) 


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https://github.com/llvm/llvm-project/pull/208253