[mutter/wip/laney/ffloat-store-x86] build: Compile with `-ffloat-store` on x86 (32 bit)

[Iain Lane <iainl src gnome org>]

commit b47d25ab7ddc8244bb008b17a073720953ac17ea
Author: Iain Lane <laney debian org>
Date:   Mon Sep 9 10:17:22 2019 +0100

    build: Compile with `-ffloat-store` on x86 (32 bit)
    
    GCC's manpage says that this flag does the following:
    
      Do not store floating-point variables in registers, and inhibit other
      options that might change whether a floating-point value is taken from
      a register or memory.
    
      This option prevents undesirable excess precision on machines such as
      the 68000 where the floating registers (of the 68881) keep more
      precision than a "double" is supposed to have.  Similarly for the x86
      architecture.  For most programs, the excess precision does only good,
      but a few programs rely on the precise definition of IEEE floating
      point.
    
    We rely on this behaviour in our fork of clutter. When performing
    floating point computations on x86, we are getting the wrong results
    because of this architecture's use of the CPU's extended (x87, non-IEEE
    confirming) precision by default. If we enable `-ffloat-store` here,
    then we'll get the same results everywhere by storing into variables
    instead of registers. This does not remove the need to be correct when
    handling floats, but it does mean we don't need to be more correct than
    the IEEE spec requires.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/785

 meson.build | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/meson.build b/meson.build
index 472693ca4..db446406e 100644
--- a/meson.build
+++ b/meson.build
@@ -286,6 +286,9 @@ foreach function : required_functions
   endif
 endforeach
 
+if host_machine.cpu_family() == 'x86'
+  add_project_arguments('-ffloat-store', language: 'c')
+endif
 add_project_arguments('-D_GNU_SOURCE', language: 'c')
 
 buildtype = get_option('buildtype')