pixman: Branch 'master'

[GitLab Mirror]

 meson.build |    6 ++++++
 1 file changed, 6 insertions(+)

New commits:
commit c2fe1568ff00cba0c71667e6ba8ce1549ead2385
Author: Tom Stellard <tstellar at redhat.com>
Date:   Mon May 11 22:17:08 2020 +0000

    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

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']
 )