[Mesa-dev] [PATCH] i965/blorp_clear: Use memcpy instead of assignment to copy clear value

[Neil Roberts]

Hi,

After the looking at the problem in bug 81150 I was wondering if we
have the same problem when using glClear with integer values. Sure
enough I can trigger a similar bug on 32-bit builds with optimisations
using a piglit test which I've posted here:

http://lists.freedesktop.org/archives/piglit/2014-August/012144.html

- Neil

------- >8 --------------- (use git am --scissors to automatically chop here)

Similar to the problem described in 2c50212b14da27de4e3, if we copy the clear
value through a regular assignment via a floating point value, then if an
integer clear value is being used that happens to contain a signalling NaN
value then it would get converted to a quiet NaN when stored via the x87
floating-point registers. This would corrupt the integer value. Instead we
should use a memcpy to ensure the exact bit representation is preserved.

This bug can be triggered on 32-bit builds with optimisations by using an
integer clear color with a value like 0x7f817f81.
---
 src/mesa/drivers/dri/i965/brw_blorp_clear.cpp | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp b/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp
index ffbcd1a..8db0837 100644
--- a/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp
+++ b/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp
@@ -202,12 +202,7 @@ brw_blorp_clear_params::brw_blorp_clear_params(struct brw_context *brw,
       y1 = rb->Height - fb->_Ymin;
    }
 
-   float *push_consts = (float *)&wm_push_consts;
-
-   push_consts[0] = ctx->Color.ClearColor.f[0];
-   push_consts[1] = ctx->Color.ClearColor.f[1];
-   push_consts[2] = ctx->Color.ClearColor.f[2];
-   push_consts[3] = ctx->Color.ClearColor.f[3];
+   memcpy(&wm_push_consts.dst_x0, ctx->Color.ClearColor.f, sizeof(float) * 4);
 
    use_wm_prog = true;
 
@@ -250,7 +245,7 @@ brw_blorp_clear_params::brw_blorp_clear_params(struct brw_context *brw,
    if (irb->mt->fast_clear_state != INTEL_FAST_CLEAR_STATE_NO_MCS &&
        !partial_clear && wm_prog_key.use_simd16_replicated_data &&
        is_color_fast_clear_compatible(brw, format, &ctx->Color.ClearColor)) {
-      memset(push_consts, 0xff, 4*sizeof(float));
+      memset(&wm_push_consts, 0xff, 4*sizeof(float));
       fast_clear_op = GEN7_FAST_CLEAR_OP_FAST_CLEAR;
 
       /* Figure out what the clear rectangle needs to be aligned to, and how
-- 
1.9.3