[Mesa-dev] [PATCH] i965: Fix computation of abs(-x) in FS

[Paul Berry]

When updating a register reference to reflect the fact that we were
taking its absolute value, the fragment shader back-end failed to
clear the negate flag, resulting in abs(-x) getting computed as
-abs(x).

I also found (and fixed) a similar problem in brw_eu.h, but I'm not
aware of an actual manifestation of that problem.

Fixes piglit test glsl-fs-abs-neg-with-intermediate.
---
 src/mesa/drivers/dri/i965/brw_eu.h   |    1 +
 src/mesa/drivers/dri/i965/brw_fs.cpp |    5 ++++-
 2 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_eu.h b/src/mesa/drivers/dri/i965/brw_eu.h
index 31334ce..a3951a6 100644
--- a/src/mesa/drivers/dri/i965/brw_eu.h
+++ b/src/mesa/drivers/dri/i965/brw_eu.h
@@ -698,6 +698,7 @@ static INLINE struct brw_reg negate( struct brw_reg reg )
 static INLINE struct brw_reg brw_abs( struct brw_reg reg )
 {
    reg.abs = 1;
+   reg.negate = 0;
    return reg;
 }
 
diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index e073eaa..7581d2b 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -1363,8 +1363,11 @@ fs_visitor::register_coalesce()
 		scan_inst->src[i].reg == inst->dst.reg &&
 		scan_inst->src[i].reg_offset == inst->dst.reg_offset) {
 	       fs_reg new_src = inst->src[0];
+               if (scan_inst->src[i].abs) {
+                  new_src.negate = 0;
+                  new_src.abs = 1;
+               }
 	       new_src.negate ^= scan_inst->src[i].negate;
-	       new_src.abs |= scan_inst->src[i].abs;
 	       scan_inst->src[i] = new_src;
 	    }
 	 }
-- 
1.7.6.4