[Mesa-dev] [PATCH] meson: enable -fmerge-all-constants by default

[Konstantin Kharlamov]

This is a standard non-conforming optimization, which, quoting,
"…considers e.g. even constant initialized arrays or initialized
constant variables with integral or floating-point types. Languages like
C or C++ require each variable, including multiple instances of the same
variable in recursive calls, to have distinct locations, so using this
option results in non-conforming behavior."

Having the same or distinct addresses of constants in recursive calls is
too obscure behavior to make any use of. More over, until 4 April 2018
clang have had this option enabled by default¹, and nobody ever noticed
until kernel build fail this year (having a low-level hackery in kernel
surprises no one), resulting in PR disabling the option by default.

As for Mesa: I personally have been using the option for release builds
since forever, never had any problems.

Real world significance, sizes of binaries in bytes:

default                                      | -fmerge-all-constants                        | diff
-----------------------------------------------------------------------------------------------------
 8048024 <-> bellagio/libomx_mesa.so         |  7867800 <-> bellagio/libomx_mesa.so         | -180224
10209120 <-> d3d/d3dadapter9.so.1.0.0        | 10016608 <-> d3d/d3dadapter9.so.1.0.0        | -192512
14464936 <-> dri/i915_dri.so                 | 14383016 <-> dri/i915_dri.so                 | -81920
16894344 <-> dri/kms_swrast_dri.so           | 16644488 <-> dri/kms_swrast_dri.so           | -249856
 8077560 <-> dri/nouveau_drv_video.so        |  7893240 <-> dri/nouveau_drv_video.so        | -184320
 3882872 <-> gallium-pipe/pipe_nouveau.so    |  3870584 <-> gallium-pipe/pipe_nouveau.so    | -12288
 2511160 <-> gallium-pipe/pipe_r300.so       |  2502968 <-> gallium-pipe/pipe_r300.so       | -8192
 2818520 <-> gallium-pipe/pipe_r600.so       |  2810328 <-> gallium-pipe/pipe_r600.so       | -8192
 4123056 <-> gallium-pipe/pipe_radeonsi.so   |  3955120 <-> gallium-pipe/pipe_radeonsi.so   | -167936
 2413936 <-> gallium-pipe/pipe_swrast.so     |  2401648 <-> gallium-pipe/pipe_swrast.so     | -12288
 2701360 <-> gallium-pipe/pipe_vmwgfx.so     |  2689072 <-> gallium-pipe/pipe_vmwgfx.so     | -12288
  582496 <-> libGLX_mesa.so.0.0.0            |   578400 <-> libGLX_mesa.so.0.0.0            | -4096
23451656 <-> libMesaOpenCL.so.1.0.0          | 23447560 <-> libMesaOpenCL.so.1.0.0          | -4096
10370344 <-> libOSMesa.so.8.0.0              | 10300712 <-> libOSMesa.so.8.0.0              | -69632
 4766032 <-> libvulkan_intel.so              |  4745552 <-> libvulkan_intel.so              | -20480
 2761376 <-> libvulkan_radeon.so             |  2753184 <-> libvulkan_radeon.so             | -8192
 4703776 <-> libxatracker.so.2.4.0           |  4687392 <-> libxatracker.so.2.4.0           | -16384
 8510392 <-> vdpau/libvdpau_nouveau.so.1.0.0 |  8326072 <-> vdpau/libvdpau_nouveau.so.1.0.0 | -184320

For consistency it's enabled for both debug and release builds.

1: https://reviews.llvm.org/D45289

Signed-off-by: Konstantin Kharlamov <Hi-Angel at yandex.ru>
---
 meson.build | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/meson.build b/meson.build
index e05645cbf3..36016cf959 100644
--- a/meson.build
+++ b/meson.build
@@ -794,6 +794,11 @@ foreach a : ['-Wall', '-fno-math-errno', '-fno-trapping-math',
   endif
 endforeach
 
+if ['gcc', 'clang'].contains(cc.get_id())
+  c_args += '-fmerge-all-constants'
+  cpp_args += '-fmerge-all-constants'
+endif
+
 # For some reason, the test for -Wno-foo always succeeds with gcc, even if the
 # option is not supported. Hence, check for -Wfoo instead.
 
-- 
2.18.0