Source: cura-engine Version: 5.0.0-1 Severity: serious Tags: ftbfs Justification: fails to build from source (but built successfully in the past) Usertags: i686 Forwarded: https://github.com/Ultimaker/CuraEngine/issues/1192 X-Debbugs-Cc: onit...@gmail.com
On i686, CuraEngine 5.x fails to build due to failing unit tests. This is a longstanding issue, going back to 4.4, where it was fixed by adding a larger tolerance to test values. However, the issue was not investigated thoroughly and returns in 5.0 with more failing unit tests. The root cause of these failures are rounding errors on i686, where the x87 FPU produces different results than floating point units in other processors. These differences are tiny, and usually not more than a few ULPs. CuraEngine uses integer math in most places, but resorts to double-precision floating-point calculations in certain cases. Afterwards, the results are truncated to 64-bit integers (C type long long), and subsequent calculation is done on the integer values. Truncation (aka round-toward-zero) is often ok and works fine on amd64 (SSE2 floating-point math) and other CPUs, but produces different results on the x87 FPU. When truncating, these produce off-by-one errors in many cases, and the these errors can accumulate and lead to huge differences in the unit tests. By strategically adding explicit rounding (round-to-nearest) in the right places, these errors can be eliminated. While this will produce subtly different results in some cases, it is arguably more correct than always truncating. And at least on amd64, there is no performance difference between truncation and rounding: The SSE2 CVTTSD2SI and CVTSD2SI instructions have the same performance. -- System Information: Debian Release: trixie/sid APT prefers testing APT policy: (990, 'testing'), (500, 'unstable-debug'), (500, 'testing-debug'), (300, 'unstable'), (1, 'experimental-debug'), (1, 'experimental') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 6.3.0-2-amd64 (SMP w/4 CPU threads; PREEMPT) Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en Shell: /bin/sh linked to /usr/bin/dash Init: systemd (via /run/systemd/system) LSM: AppArmor: enabled -- no debconf information
