Clang/ARM64 compilation issue

README.md

Compilation issue with clang 14 on ARM64

To reproduce:

• Use an ARM64 computer (tested on M1 mac and on Raspberry Pi model 4B)

  • On Raspberry pi, I used Raspbian:bookworm
  • On M1 mac, I used docker run --rm -ti -v $(pwd):/demo -w /demo debian:bookworm /bin/bash (which shares the local directory into the container, assumes you are running this from the cloned gist)

• On your computer of choice, run apt update && apt install -y build-essential clang-13 clang

• Observe gcc produces correct results: gcc ./min-repro-sample.c && ./a.out

  The number printed here is expected to be 0.0.
  GCC (all versions tested) produces the correct result.
  Clang 14.0.6 introduces small numerical error, producing a value > 0
  The latest version of clang tested producing the correct result is clang 13.0.1 (from debian:bookwork clang-13)
  result: 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000
  
  On clang 14.0.6, the calculated result below is a workaround that produces the expected 0.0
  result: 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000
  

• Observe clang-13 produces correct results: clang-13 ./min-repro-sample.c && ./a.out

  The number printed here is expected to be 0.0.
  GCC (all versions tested) produces the correct result.
  Clang 14.0.6 introduces small numerical error, producing a value > 0
  The latest version of clang tested producing the correct result is clang 13.0.1 (from debian:bookwork clang-13)
  result: 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000
  
  On clang 14.0.6, the calculated result below is a workaround that produces the expected 0.0
  result: 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000
  

• Observe clang-14 produces unexpected results: clang-14 ./min-repro-sample.c && ./a.out

  The number printed here is expected to be 0.0.
  GCC (all versions tested) produces the correct result.
  Clang 14.0.6 introduces small numerical error, producing a value > 0
  The latest version of clang tested producing the correct result is clang 13.0.1 (from debian:bookwork clang-13)
  result: 0.00000000000000004058614543027810527553055411362964847252091191848188955759724195
  
  On clang 14.0.6, the calculated result below is a workaround that produces the expected 0.0
  result: 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000
  

min-repro-sample.c

#include <stdio.h>

int main() {
    double previous_height = 1.0;
    double r_v = 2.0;
    double slope_distance = 1.4142135623730951;
    double target_distance = 2.8284271247461903;
    
    printf("The number printed here is expected to be 0.0.\n");
    printf("GCC (all versions tested) produces the correct result.\n");
    printf("Clang 14.0.6 introduces small numerical error, producing a value > 0\n");
    printf("The latest version of clang tested producing the correct result is clang 13.0.1 (from debian:bookworm clang-13)\n");
    printf("result: %.80f\n", ((((previous_height-r_v)/slope_distance) * target_distance) + r_v));
    
    printf("\n");
    printf("On clang 14.0.6, the calculated result below is a workaround that produces the expected 0.0\n");
    double mult = (((previous_height-r_v)/slope_distance) * target_distance);
    double added = mult + r_v;
    printf("result: %.80f\n", added);
    return 0;
}