Roman Lebedev 5d534d8259 [llvm-exegesis] Loop unrolling for loop snippet repetitor mode ... I really needed this, like, factually, yesterday, when verifying dependency breaking idioms for AMD Zen 3 scheduler model. Consider the following example: ``` $ ./bin/llvm-exegesis --mode=inverse_throughput --snippets-file=/tmp/snippet.s --num-repetitions=1000000 --repetition-mode=duplicate Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-4a7e50.o --- mode: inverse_throughput key: instructions: - 'VPXORYrr YMM0 YMM0 YMM0' config: '' register_initial_values: [] cpu_name: znver3 llvm_triple: x86_64-unknown-linux-gnu num_repetitions: 1000000 measurements: - { key: inverse_throughput, value: 0.31025, per_snippet_value: 0.31025 } error: '' info: '' assembled_snippet: C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C5FDEFC0C3 ... ``` What does it tell us? So wait, it can only execute ~3 x86 AVX YMM PXOR zero-idioms per cycle? That doesn't seem right. That's even less than there are pipes supporting this type of op. Now, second example: ``` $ ./bin/llvm-exegesis --mode=inverse_throughput --snippets-file=/tmp/snippet.s --num-repetitions=1000000 --repetition-mode=loop Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-2418b5.o --- mode: inverse_throughput key: instructions: - 'VPXORYrr YMM0 YMM0 YMM0' config: '' register_initial_values: [] cpu_name: znver3 llvm_triple: x86_64-unknown-linux-gnu num_repetitions: 1000000 measurements: - { key: inverse_throughput, value: 1.00011, per_snippet_value: 1.00011 } error: '' info: '' assembled_snippet: 49B80800000000000000C5FDEFC0C5FDEFC04983C0FF75F2C3 ... ``` Now that's just worse. Due to the looping, the throughput completely plummeted, and now we can only do a single instruction/cycle!? That's not great. And final example: ``` $ ./bin/llvm-exegesis --mode=inverse_throughput --snippets-file=/tmp/snippet.s --num-repetitions=1000000 --repetition-mode=loop --loop-body-size=1000 Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-c402e2.o --- mode: inverse_throughput key: instructions: - 'VPXORYrr YMM0 YMM0 YMM0' config: '' register_initial_values: [] cpu_name: znver3 llvm_triple: x86_64-unknown-linux-gnu num_repetitions: 1000000 measurements: - { key: inverse_throughput, value: 0.167087, per_snippet_value: 0.167087 } error: '' info: '' assembled_snippet: 49B80800000000000000C5FDEFC0C5FDEFC04983C0FF75F2C3 ... ``` So if we merge the previous two approaches, do duplicate this single-instruction snippet 1000x (loop-body-size/instruction count in snippet), and run a loop with 1000 iterations over that duplicated/unrolled snippet, the measured throughput goes through the roof, up to 5.9 instructions/cycle, which finally tells us that this idiom is zero-cycle! Reviewed By: courbet Differential Revision: https://reviews.llvm.org/D102522		2021-05-25 12:08:27 +03:00
..
llvm-cfi-verify	[llvm][NFC] Add missing 'override's in unittests/	2020-07-17 17:35:59 -07:00
llvm-exegesis	[llvm-exegesis] Loop unrolling for loop snippet repetitor mode	2021-05-25 12:08:27 +03:00
llvm-profgen	[CSSPGO][llvm-profgen] Compress recursive cycles in calling context	2021-02-03 22:16:07 -08:00
CMakeLists.txt	[CSSPGO][llvm-profgen] Compress recursive cycles in calling context	2021-02-03 22:16:07 -08:00