| Issue |
165809
|
| Summary |
[Flang][AArch64] Missed optimization opportunities with `unsafe-math-optimizations`
|
| Labels |
missed-optimization,
floating-point
|
| Assignees |
|
| Reporter |
yus3710-fj
|
Duplicate of #165514
The execution time of Flang-compiled version of `blts` subroutine in `blts.f90` from LU in [NAS Parallel Benchmarks](https://www.nas.nasa.gov/software/npb.html) 3.4.3 is significantly slower than its GFortran-compiled one.
We measured the execution time using the `perf` command on Rocky Linux, running on an NVIDIA GRACE processor.
The sampling frequency for `perf` was set to 97, as detailed in the command below.
We used Flang built from commit `3e1d4d4144cc9d28ccd85cf49d6fc836c38ffbaa`.
The GFortran version was 15.2.0.
All benchmarks were conducted by executing following command:
```bash
perf record --call-graph fp -F 97 -o data /path/to/compiled/binary
```
We observed that the execution time of Flang's compilation result, when compiled with the `-Ofast -mcpu=neoverse-v2` option, was 167% of GFortran's execution time.
While the benchmark does not specify these options, we use them to obtain the optimal performance.
We found two factors: Flang's failure to apply adequate vectorization (#49896, #46522) and fast-math optimizations.
Specifically, Flang missed `reciprocal-math` optimization opportunities.
We compiled `blts.f90` from LU in NAS Parallel Benchmarks with the following compile options:
```bash
-g -Ofast -mcpu=neoverse-v2 -fno-tree-vectorize -fno-fast-math -freciprocal-math
```
Note: Several parameters were specified to mitigate the influence of other factors on performance.
The execution time of Flang-compiled executable is 124% of GFortran's.
We checked the assembly code by toggling `-freciprocal-math` and found no diffs in Flang's output, whereas GFortran's output did show changes.
Additionally, while GFortran can optimize by specifying both `-fno-signed-zeros` and `-fno-trapping-math`, the latter (`-fno-trapping-math`) is not implemented in Flang.
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