[all-commits] [llvm/llvm-project] 111d27: [libc][math] Implement double precision log2 funct...

[lntue via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 111d27484132c0692c214880576dc4a37fd6d645
      https://github.com/llvm/llvm-project/commit/111d27484132c0692c214880576dc4a37fd6d645
  Author: Tue Ly <lntue at google.com>
  Date:   2023-05-23 (Tue, 23 May 2023)

  Changed paths:
    M libc/config/darwin/arm/entrypoints.txt
    M libc/config/linux/aarch64/entrypoints.txt
    M libc/config/linux/x86_64/entrypoints.txt
    M libc/config/windows/entrypoints.txt
    M libc/spec/stdc.td
    M libc/src/math/CMakeLists.txt
    M libc/src/math/generic/CMakeLists.txt
    A libc/src/math/generic/log2.cpp
    A libc/src/math/log2.h
    M libc/src/string/memory_utils/utils.h
    M libc/test/src/math/CMakeLists.txt
    A libc/test/src/math/log2_test.cpp
    M libc/test/src/string/memory_utils/utils_test.cpp

  Log Message:
  -----------
  [libc][math] Implement double precision log2 function correctly rounded to all rounding modes.

Implement double precision log2 function correctly rounded to all
rounding modes.

See https://reviews.llvm.org/D150014 for a more detail description of the algorithm.

**Performance**

  - For `0.5 <= x <= 2`, the fast pass hitting rate is about 99.91%.

  - Reciprocal throughput from CORE-MATH's perf tool on Ryzen 5900X:
```
$ ./perf.sh log2
GNU libc version: 2.35
GNU libc release: stable

-- CORE-MATH reciprocal throughput -- with FMA
[####################] 100 %
Ntrial = 20 ; Min = 15.458 + 0.204 clc/call; Median-Min = 0.224 clc/call; Max = 15.867 clc/call;

-- CORE-MATH reciprocal throughput -- without FMA (-march=x86-64-v2)
[####################] 100 %
Ntrial = 20 ; Min = 23.711 + 0.524 clc/call; Median-Min = 0.443 clc/call; Max = 25.307 clc/call;

-- System LIBC reciprocal throughput --
[####################] 100 %
Ntrial = 20 ; Min = 14.807 + 0.199 clc/call; Median-Min = 0.211 clc/call; Max = 15.137 clc/call;

-- LIBC reciprocal throughput -- with FMA
[####################] 100 %
Ntrial = 20 ; Min = 17.666 + 0.274 clc/call; Median-Min = 0.298 clc/call; Max = 18.531 clc/call;

-- LIBC reciprocal throughput -- without FMA
[####################] 100 %
Ntrial = 20 ; Min = 26.534 + 0.418 clc/call; Median-Min = 0.462 clc/call; Max = 27.327 clc/call;

```
  - Latency from CORE-MATH's perf tool on Ryzen 5900X:
```
$ ./perf.sh log2 --latency
GNU libc version: 2.35
GNU libc release: stable

-- CORE-MATH latency -- with FMA
[####################] 100 %
Ntrial = 20 ; Min = 46.048 + 1.643 clc/call; Median-Min = 1.694 clc/call; Max = 48.018 clc/call;

-- CORE-MATH latency -- without FMA (-march=x86-64-v2)
[####################] 100 %
Ntrial = 20 ; Min = 62.333 + 0.138 clc/call; Median-Min = 0.119 clc/call; Max = 62.583 clc/call;

-- System LIBC latency --
[####################] 100 %
Ntrial = 20 ; Min = 45.206 + 1.503 clc/call; Median-Min = 1.467 clc/call; Max = 47.229 clc/call;

-- LIBC latency -- with FMA
[####################] 100 %
Ntrial = 20 ; Min = 43.042 + 0.454 clc/call; Median-Min = 0.484 clc/call; Max = 43.912 clc/call;

-- LIBC latency -- without FMA
[####################] 100 %
Ntrial = 20 ; Min = 57.016 + 1.636 clc/call; Median-Min = 1.655 clc/call; Max = 58.816 clc/call;
```
  - Accurate pass latency:
```
$ ./perf.sh log2 --latency --simple_stat
GNU libc version: 2.35
GNU libc release: stable

-- CORE-MATH latency -- with FMA
177.632

-- CORE-MATH latency -- without FMA (-march=x86-64-v2)
231.332

-- LIBC latency -- with FMA
459.751

-- LIBC latency -- without FMA
463.850
```

Reviewed By: zimmermann6

Differential Revision: https://reviews.llvm.org/D150374