Bug 1700717 - Build SWGL with -ffast-math. r=lsalzman

Most GLSL compilers will do the kinds of optimizations that
fast-math allows so we should take advantage of them too.

Differential Revision: https://phabricator.services.mozilla.com/D109657

gfx/wr/swgl/build.rs

@@ -176,6 +176,22 @@ fn main() {
         if tool.args().contains(&"-Oz".into()) {
             build.flag("-O2");
         }
+
+        // Most GLSL compilers assume something like fast-math so we turn it on.
+        // However, reciprocal division makes it so 1/1 = 0.999994 which can produce a lot of fuzz
+        // in reftests and the use of reciprocal instructions usually involves a refinement step
+        // which bloats our already bloated code. Further, our shader code is sufficiently parallel
+        // that we're more likely to be throughput bound vs latency bound. Having fewer
+        // instructions makes things easier on the processor and in places where it matters we can
+        // probably explicitly use reciprocal instructions and avoid the refinement step.
+        if tool.is_like_msvc() {
+            build.flag("/fp:fast")
+                 .flag("-Xclang")
+                 .flag("-mrecip=none");
+        } else {
+            build.flag("-ffast-math")
+                 .flag("-mrecip=none");
+        }
     }
 
     build.file("src/gl.cc")

layout/reftests/border-image/reftest.list

@@ -73,7 +73,7 @@ fuzzy(0-1,0-19200) fails-if(OSX) fuzzy-if(skiaContent,0-3,0-20000) fuzzy-if(webr
 fuzzy(0-1,0-912) fuzzy-if(webrender,0-3,0-3008) == border-image-repeating-radial-gradient.html border-image-repeating-radial-gradient-ref.html
 fuzzy(0-1,0-1720) fuzzy-if(skiaContent,0-3,0-1044) fuzzy-if(webrender,0-3,0-62078) == border-image-repeating-radial-gradient-slice-1.html border-image-repeating-radial-gradient-slice-1-ref.html
 fuzzy(0-1,0-2119) fuzzy-if(skiaContent,0-3,0-936) fuzzy-if(webrender,0-3,0-40536) == border-image-repeating-radial-gradient-slice-2.html border-image-repeating-radial-gradient-slice-2-ref.html
-fuzzy(0-1,0-3170) fuzzy-if(skiaContent,0-3,0-1794) fuzzy-if(webrender&&!geckoview,1-3,30-12193) == border-image-repeating-radial-gradient-slice-fill-1.html border-image-repeating-radial-gradient-slice-fill-1-ref.html
+fuzzy(0-1,0-3170) fuzzy-if(skiaContent,0-3,0-1794) fuzzy-if(webrender&&!geckoview,1-3,30-12194) == border-image-repeating-radial-gradient-slice-fill-1.html border-image-repeating-radial-gradient-slice-fill-1-ref.html
 fuzzy(0-1,0-3281) fails-if(OSX) fuzzy-if(skiaContent,0-3,0-1435) fuzzy-if(webrender&&!geckoview,1-2,20-8000) == border-image-repeating-radial-gradient-slice-fill-2.html border-image-repeating-radial-gradient-slice-fill-2-ref.html
 fuzzy(0-1,0-1328) fuzzy-if(webrender,0-3,0-7441) == border-image-repeating-radial-gradient-width.html border-image-repeating-radial-gradient-width-ref.html
 fuzzy(0-4,0-21628) fails-if(OSX) fuzzy-if(webrender,0-8,0-99728) == border-image-repeating-radial-gradient-slice-width.html border-image-repeating-radial-gradient-slice-width-ref.html