[Mesa-dev] [PATCH v2 06/18] intel/compiler: fix brw_imm_w for negative 16-bit integers

[Iago Toral Quiroga]

From: Jose Maria Casanova Crespo <jmcasanova at igalia.com>

16-bit immediates need to replicate the 16-bit immediate value
in both words of the 32-bit value. This needs to be careful
to avoid sign-extension, which the previous implementation was
not handling properly.

For example, with the previous implementation, storing the value
-3 would generate imm.d = 0xfffffffd due to signed integer sign
extension, which is not correct. Instead, we should cast to
unsigned, which gives us the correct result: imm.ud = 0xfffdfffd.

We only had a couple of cases hitting this path in the driver
until now, one with value -1, which would work since all bits are
one in this case, and another with value -2 in brw_clip_tri(),
which would hit the aforementioned issue (this case only affects
gen4 although we are not aware of whether this was causing an
actual bug somewhere).
---
 src/intel/compiler/brw_reg.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/intel/compiler/brw_reg.h b/src/intel/compiler/brw_reg.h
index dff9b970b2..0084a78af6 100644
--- a/src/intel/compiler/brw_reg.h
+++ b/src/intel/compiler/brw_reg.h
@@ -705,7 +705,7 @@ static inline struct brw_reg
 brw_imm_w(int16_t w)
 {
    struct brw_reg imm = brw_imm_reg(BRW_REGISTER_TYPE_W);
-   imm.d = w | (w << 16);
+   imm.ud = (uint16_t)w | ((uint16_t)w << 16);
    return imm;
 }
 
-- 
2.14.1