GCC vs. Clang (Chpl compiler) Optimization Issue

Summary of Problem

I was working on another issue when I encountered this issue between Clang (Chpl) and GCC. It may be out of scope for the Chapel compiler, but the compiler writers maybe interested in this issue.

When I was writing a sin function calculator I saw two different performances between the GCC compiler and Clang (Chpl) compiler. This was discovered with working with C-extern functions and replicated separately with the Clang compiler. (Version 11.0.0 and 14.0.6) I am not sure how much the Chapel compiler is doing when it comes to external C code or if the issue is all with the LLVM back-end.

Please feel free to close this ticket if you think this should be submitted further upstream.

Steps to Reproduce

Adding a return statement is the trigger between the performance. This was noticed when returning a value from a C-extern function.

Source Code: Optimized/Fast Code with Return Statement:

#include <stdint.h>
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>

int main() {

    time_t t;
    srand((unsigned )time(&t));
    int size = 256;
    int iterations = 100000000;
    float answer = 0;

    for (int i=0; i < iterations; i++) {
        int random_number = rand() % (size-1);
        answer = 2 * 6.28 * sin(2.0 * 3.1415927 * (float) random_number / (float)size);
    }
}

With Return Statement

#include <stdint.h>
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>

int main() {

    time_t t;
    srand((unsigned) time(&t));
    int size = 256;
    int iterations = 100000000;
    float answer = 0;

    for (int i = 0; i < iterations; i++) {
        int random_number = rand() % (size-1);
        answer = 2 * 6.28 * sin(2.0 * 3.1415927 * (float) random_number / (float) size);
    }

    return answer;
}

Compile command: gcc sin.c -O3 -lm clang sin.c -O3 -lm

Execution command: time ./sin.x

Time for GCC w/o return: 0m0.441s Time for Clang w/o return: 0m0.537s

Time for GCC with return: 0m0.445s Time for Clang with return: 0m2.317s

Configuration Information

Output of gcc --version: gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-15)

Output of clang --version:

clang version 14.0.6 (Red Hat 14.0.6-1.module+el8.7.0+1080+d88dc670)
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Output of chpl --version:

chpl version 1.30.0
built with LLVM version 14.0.6
Copyright 2020-2023 Hewlett Packard Enterprise Development LP
Copyright 2004-2019 Cray Inc.

Output of $CHPL_HOME/util/printchplenv --anonymize:

CHPL_TARGET_PLATFORM: linux64
CHPL_TARGET_COMPILER: llvm
CHPL_TARGET_ARCH: x86_64
CHPL_TARGET_CPU: native *
CHPL_LOCALE_MODEL: flat
CHPL_COMM: gasnet *
CHPL_COMM_SUBSTRATE: udp *
CHPL_GASNET_SEGMENT: everything
CHPL_TASKS: qthreads
CHPL_LAUNCHER: amudprun
CHPL_TIMERS: generic
CHPL_UNWIND: none
CHPL_MEM: jemalloc
CHPL_ATOMICS: cstdlib
CHPL_NETWORK_ATOMICS: none
CHPL_GMP: bundled
CHPL_HWLOC: bundled
CHPL_RE2: bundled
CHPL_LLVM: system
CHPL_AUX_FILESYS: none

chapel-lang / chapel