Mozilla developers on the GCC mailing list have been expressing what they describe as a "massive performance regression" and "what might be the biggest compiler-upgrade-related performance difference we've seen at Mozilla." The Mozilla developers have upgraded from GCC 4.3 to GCC 4.5, which was released in April
, and now they are experiencing massive slowdowns.
. The start of the mailing list discussion can be found here
Because of these performance shortcomings on GCC 4.5, Mozilla is reconsidering their switch to this newest compiler release. At least because Mozilla has an automated benchmarking infrastructure in place, they were able to spot this issue timely themselves rather than waiting for us or others to spot the problem.
Initial thoughts on these major regressions caused by GCC are the Mozilla developers building Firefox with flags for size optimizations rather than speed and possible regressions from code inlining. Mozilla was originally switching to GCC 4.5 due to its newly-introduced plug-in support and the ability to build Firefox with PGO (Profile-Guided Optimizations).
Following the release of GCC 4.5.0 in mid-April we published our own GCC 4.5 benchmarks
and GCC vs. Clang/LLVM benchmarks