From 5dfb085ff325df3dbefda515f06106469babbefc Mon Sep 17 00:00:00 2001 From: Iago Toral Quiroga Date: Tue, 20 Jan 2015 17:09:59 +0100 Subject: 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 --- src/glsl/ir_optimization.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'src/glsl/ir_optimization.h') diff --git a/src/glsl/ir_optimization.h b/src/glsl/ir_optimization.h index 34e0b4b947d..912d9103e17 100644 --- a/src/glsl/ir_optimization.h +++ b/src/glsl/ir_optimization.h @@ -34,7 +34,7 @@ #define EXP_TO_EXP2 0x04 #define POW_TO_EXP2 0x08 #define LOG_TO_LOG2 0x10 -#define MOD_TO_FRACT 0x20 +#define MOD_TO_FLOOR 0x20 #define INT_DIV_TO_MUL_RCP 0x40 #define BITFIELD_INSERT_TO_BFM_BFI 0x80 #define LDEXP_TO_ARITH 0x100 -- cgit v1.2.3