From 10ff6772c8054aea12ac0f08e2e3898fd4a7f76b Mon Sep 17 00:00:00 2001 From: Kenneth Graunke Date: Wed, 24 Oct 2012 21:16:46 -0700 Subject: [PATCH] i965/vs: Don't lose the MRF writemask when doing compute-to-MRF. Consider the following code sequence: mul(8) g4<1>F g1<0,4,1>.wzwwF g3<4,4,1>.wzwwF mov.sat(8) m1<1>.xyF g4<4,4,1>F mul(8) g4<1>F g1<0,4,1>.xxyxF g3<4,4,1>.xxyxF mov.sat(8) m1<1>.zwF g4<4,4,1>F The compute-to-MRF pass will discover the first mov.sat and attempt to replace it by rewriting earlier instructions. Everything works out, so it replaces scan_inst's destination file, reg, and reg_offset, resulting in: mul(8) m1<1>F g1<0,4,1>.wzwwF g3<4,4,1>.wzwwF mul(8) g4<1>F g1<0,4,1>.xxyxF g3<4,4,1>.xxyxF mov.sat(8) m1<1>.zwF g4<4,4,1>F Unfortunately, it loses the .xy writemask on the mov.sat's MRF destination. While this doesn't pose an immediate problem, it then proceeds to transform the second mov.sat, resulting in: mul(8) m1<1>F g1<0,4,1>.wzwwF g3<4,4,1>.wzwwF mul(8) m1<1>F g1<0,4,1>.xxyxF g3<4,4,1>.xxyxF Instead of writing both halves of the vector (like the original code), it overwrites the full vector both times, clobbering the desired .xy values. When encountering a MOV, the compute-to-MRF code scans for instructions which generate channels of the MOV source. It ensures that all necessary channels are available (possibly written by several instructions). In this case, *more* channels are available than necessary, so we want to take the subset that's actually used. Taking the bitwise and of both writemasks should accomplish that. This was discovered by analyzing an ARB_vertex_program test (glean/vertProg1/MUL test (with swizzle and masking)) with my new Mesa IR -> Vec4 IR translator code. However, it should be possible with GLSL programs as well. NOTE: This is a candidate for stable release branches. Signed-off-by: Kenneth Graunke Reviewed-by: Eric Anholt --- src/mesa/drivers/dri/i965/brw_vec4.cpp | 1 + 1 file changed, 1 insertion(+) diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp index 5c52d3a148c..10a8310ff88 100644 --- a/src/mesa/drivers/dri/i965/brw_vec4.cpp +++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp @@ -754,6 +754,7 @@ vec4_visitor::opt_compute_to_mrf() scan_inst->dst.file = MRF; scan_inst->dst.reg = mrf; scan_inst->dst.reg_offset = 0; + scan_inst->dst.writemask &= inst->dst.writemask; scan_inst->saturate |= inst->saturate; } scan_inst = (vec4_instruction *)scan_inst->next;