Mesa (master): gallivm: implement the correct version of LRP
Marek Olšák
mareko at kemper.freedesktop.org
Sat Oct 17 19:47:31 UTC 2015
Module: Mesa
Branch: master
Commit: 529c5e77402012aa0c0a11ee71a1a65e51edb496
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=529c5e77402012aa0c0a11ee71a1a65e51edb496
Author: Marek Olšák <marek.olsak at amd.com>
Date: Sat Oct 10 22:05:58 2015 +0200
gallivm: implement the correct version of LRP
The previous version has precision issues. This can be a problem
with tessellation. Sadly, I can't find the article where I read it
anymore. I'm not sure if the unsafe-fp-math flag would be enough to revert
this.
v2: added the comment
---
src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
index 0ad78b0..3d5e2cb 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
@@ -538,12 +538,19 @@ lrp_emit(
struct lp_build_tgsi_context * bld_base,
struct lp_build_emit_data * emit_data)
{
- LLVMValueRef tmp;
- tmp = lp_build_emit_llvm_binary(bld_base, TGSI_OPCODE_SUB,
- emit_data->args[1],
- emit_data->args[2]);
- emit_data->output[emit_data->chan] = lp_build_emit_llvm_ternary(bld_base,
- TGSI_OPCODE_MAD, emit_data->args[0], tmp, emit_data->args[2]);
+ struct lp_build_context *bld = &bld_base->base;
+ LLVMValueRef inv, a, b;
+
+ /* This uses the correct version: (1 - t)*a + t*b
+ *
+ * An alternative version is "a + t*(b-a)". The problem is this version
+ * doesn't return "b" for t = 1, because "a + (b-a)" isn't equal to "b"
+ * because of the floating-point rounding.
+ */
+ inv = lp_build_sub(bld, bld_base->base.one, emit_data->args[0]);
+ a = lp_build_mul(bld, emit_data->args[1], emit_data->args[0]);
+ b = lp_build_mul(bld, emit_data->args[2], inv);
+ emit_data->output[emit_data->chan] = lp_build_add(bld, a, b);
}
/* TGSI_OPCODE_MAD */
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