From c2fe1568ff00cba0c71667e6ba8ce1549ead2385 Mon Sep 17 00:00:00 2001 From: Tom Stellard Date: Mon, 11 May 2020 22:17:08 +0000 Subject: Add -ftrapping-math to default cflags This should resolve https://gitlab.freedesktop.org/pixman/pixman/-/issues/22 and make the tests pass with clang. -ftrapping-math is already the default[1] for gcc, so this should not change behavior when compiling with gcc. However, clang defaults[2] to -fno-trapping-math, so -ftrapping-math is needed to avoid floating-point expceptions when running the combiner and stress tests. The root causes of this issue is that that pixman-combine-float.c guards floating-point division operations with a FLOAT_IS_ZERO check e.g. if (FLOAT_IS_ZERO (sa)) f = 1.0f; else f = CLAMP (da / sa); With -fno-trapping-math, the compiler assumes that division will never trap, so it may re-order the division and the guard and execute the division first. In most cases, this would not be an issue, because floating-point exceptions are ignored. However, these tests call enable_divbyzero_exceptions() which causes the SIGFPE signal to be sent to the program when a divide by zero exception is raised. [1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html [2] https://clang.llvm.org/docs/UsersManual.html#controlling-floating-point-behavior --- meson.build | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/meson.build b/meson.build index 5d65417..91d6578 100644 --- a/meson.build +++ b/meson.build @@ -37,6 +37,12 @@ add_project_arguments( '-fno-strict-aliasing', '-fvisibility=hidden', '-Wundef', + # -ftrapping-math is the default for gcc, but -fno-trapping-math is the + # default for clang. The FLOAT_IS_ZERO macro is used to guard against + # floating-point exceptions, however with -fno-trapping-math, the compiler + # can reorder floating-point operations so that they occur before the guard. + # Note, this function is ignored in clang < 10.0.0. + '-ftrapping-math' ]), language : ['c'] ) -- cgit v1.2.1