[Mesa-dev] [PATCH v2] translate_sse: Preserve low bit during unsigned -> float conversion.

[Andreas Hartmetz]

From: Andreas Hartmetz <andreas.hartmetz at kdab.com>

---
 src/gallium/auxiliary/translate/translate_sse.c | 35 ++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/src/gallium/auxiliary/translate/translate_sse.c b/src/gallium/auxiliary/translate/translate_sse.c
index dace722..2a0d3c1 100644
--- a/src/gallium/auxiliary/translate/translate_sse.c
+++ b/src/gallium/auxiliary/translate/translate_sse.c
@@ -71,7 +71,7 @@ struct translate_group
 
 #define ELEMENT_BUFFER_INSTANCE_ID  1001
 
-#define NUM_CONSTS 7
+#define NUM_CONSTS 8
 
 enum
 {
@@ -81,6 +81,7 @@ enum
    CONST_INV_32767,
    CONST_INV_65535,
    CONST_INV_2147483647,
+   CONST_INV_4294967295,
    CONST_255
 };
 
@@ -92,6 +93,7 @@ static float consts[NUM_CONSTS][4] = {
    C(1.0 / 32767.0),
    C(1.0 / 65535.0),
    C(1.0 / 2147483647.0),
+   C(1.0 / 4294967295.0),
    C(255.0)
 };
 
@@ -515,6 +517,7 @@ translate_attr_convert(struct translate_sse *p,
         || a->output_format == PIPE_FORMAT_R32G32B32_FLOAT
         || a->output_format == PIPE_FORMAT_R32G32B32A32_FLOAT)) {
       struct x86_reg dataXMM = x86_make_reg(file_XMM, 0);
+      struct x86_reg dataXMM2 = x86_make_reg(file_XMM, 1);
 
       for (i = 0; i < output_desc->nr_channels; ++i) {
          if (swizzle[i] == UTIL_FORMAT_SWIZZLE_0
@@ -546,17 +549,38 @@ translate_attr_convert(struct translate_sse *p,
                 */
                sse2_punpcklbw(p->func, dataXMM, get_const(p, CONST_IDENTITY));
                sse2_punpcklbw(p->func, dataXMM, get_const(p, CONST_IDENTITY));
+               sse2_cvtdq2ps(p->func, dataXMM, dataXMM);
                break;
             case 16:
                sse2_punpcklwd(p->func, dataXMM, get_const(p, CONST_IDENTITY));
+               sse2_cvtdq2ps(p->func, dataXMM, dataXMM);
                break;
-            case 32:           /* we lose precision here */
+            case 32:           /* we lose precision if value > 2^23 - 1 */
+               /* ...but try to keep all bits for value <= 2^23 - 1 */
+               /* this could be done much smarter for 1 or 2 dwords of input
+                * on X64, using sth like "cvtsi2ss xmm0, rax" (note *R*ax) */
+
+               /* save the low bit */
+               sse2_movdqa(p->func, dataXMM2, dataXMM);
+               /* right shift & convert, losing the low bit - must clear
+                * high bit because there is no unsigned convert instruction */
                sse2_psrld_imm(p->func, dataXMM, 1);
+               sse2_cvtdq2ps(p->func, dataXMM, dataXMM);
+
+               /* convert low bit to float */
+               sse2_pslld_imm(p->func, dataXMM2, 31);
+               sse2_psrld_imm(p->func, dataXMM2, 31);
+               sse2_cvtdq2ps(p->func, dataXMM2, dataXMM2);
+
+               /* mostly undo the right-shift by 1 */
+               sse_addps(p->func, dataXMM, dataXMM);
+               /* add back the lost low bit */
+               sse_addps(p->func, dataXMM, dataXMM2);
                break;
             default:
                return FALSE;
             }
-            sse2_cvtdq2ps(p->func, dataXMM, dataXMM);
+
             if (input_desc->channel[0].normalized) {
                struct x86_reg factor;
                switch (input_desc->channel[0].size) {
@@ -567,7 +591,7 @@ translate_attr_convert(struct translate_sse *p,
                   factor = get_const(p, CONST_INV_65535);
                   break;
                case 32:
-                  factor = get_const(p, CONST_INV_2147483647);
+                  factor = get_const(p, CONST_INV_4294967295);
                   break;
                default:
                   assert(0);
@@ -579,9 +603,6 @@ translate_attr_convert(struct translate_sse *p,
                }
                sse_mulps(p->func, dataXMM, factor);
             }
-            else if (input_desc->channel[0].size == 32)
-               /* compensate for the bit we threw away to fit u32 into s32 */
-               sse_addps(p->func, dataXMM, dataXMM);
             break;
          case UTIL_FORMAT_TYPE_SIGNED:
             if (!(x86_target_caps(p->func) & X86_SSE2))
-- 
1.9.1