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authorIago Toral Quiroga <itoral@igalia.com>2015-01-20 17:09:59 +0100
committerIago Toral Quiroga <itoral@igalia.com>2015-02-03 13:19:36 +0100
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glsl: Improve precision of mod(x,y)
Currently, Mesa uses the lowering pass MOD_TO_FRACT to implement mod(x,y) as y * fract(x/y). This implementation has a down side though: it introduces precision errors due to the fract() operation. Even worse, since the result of fract() is multiplied by y, the larger y gets the larger the precision error we produce, so for large enough numbers the precision loss is significant. Some examples on i965: Operation Precision error ----------------------------------------------------- mod(-1.951171875, 1.9980468750) 0.0000000447 mod(121.57, 13.29) 0.0000023842 mod(3769.12, 321.99) 0.0000762939 mod(3769.12, 1321.99) 0.0001220703 mod(-987654.125, 123456.984375) 0.0160663128 mod( 987654.125, 123456.984375) 0.0312500000 This patch replaces the current lowering pass with a different one (MOD_TO_FLOOR) that follows the recommended implementation in the GLSL man pages: mod(x,y) = x - y * floor(x/y) This implementation eliminates the precision errors at the expense of an additional add instruction on some systems. On systems that can do negate with multiply-add in a single operation this new implementation would come at no additional cost. v2 (Ian Romanick) - Do not clone operands because when they are expressions we would be duplicating them and that can lead to suboptimal code. Fixes the following 16 dEQP tests: dEQP-GLES3.functional.shaders.builtin_functions.precision.mod.mediump_* dEQP-GLES3.functional.shaders.builtin_functions.precision.mod.highp_* Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Diffstat (limited to 'src/glsl/README')
-rw-r--r--src/glsl/README2
1 files changed, 1 insertions, 1 deletions
diff --git a/src/glsl/README b/src/glsl/README
index 2f93f12..bfcf69f 100644
--- a/src/glsl/README
+++ b/src/glsl/README
@@ -187,7 +187,7 @@ You may also need to update the backends if they will see the new expr type:
You can then use the new expression from builtins (if all backends
would rather see it), or scan the IR and convert to use your new
-expression type (see ir_mod_to_fract, for example).
+expression type (see ir_mod_to_floor, for example).
Q: How is memory management handled in the compiler?