[Mesa-dev] [PATCH] gallivm: fix broken 8-wide s3tc decoding

[Jose Fonseca]

LGTM.

Reviewed-by: Jose Fonseca <jfonseca at vmware.com>

________________________________
From: sroland at vmware.com <sroland at vmware.com>
Sent: Tuesday, May 7, 2019 03:12
To: Jose Fonseca; Brian Paul; mesa-dev at lists.freedesktop.org
Cc: Roland Scheidegger
Subject: [PATCH] gallivm: fix broken 8-wide s3tc decoding

From: Roland Scheidegger <sroland at vmware.com>

Brian noticed there was an uninitialized var for the 8-wide case and 128
bit blocks, which made it always crash. Likewise, the 64bit block case
had another crash bug due to type mismatch.
Color decode (used for all s3tc formats) also had a bogus shuffle for
this case, leading to decode artifacts.
Fix these all up, which makes the code actually work 8-wide. Note that
it's still not used - I've verified it works, and the generated assembly
does look quite a bit simpler actually (20-30% less instructions for the
s3tc decode part with avx2), however in practice it still seems to be
sligthly slower for some unknown reason (tested with openarena) on my
haswell box, so for now continue to split things into 4-wide vectors
before decoding.
---
 .../auxiliary/gallivm/lp_bld_format_s3tc.c    | 33 +++++++++----------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/src/gallium/auxiliary/gallivm/lp_bld_format_s3tc.c b/src/gallium/auxiliary/gallivm/lp_bld_format_s3tc.c
index 9561c349dad..8f6e9bec18a 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_format_s3tc.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_format_s3tc.c
@@ -77,24 +77,17 @@ lp_build_uninterleave2_half(struct gallivm_state *gallivm,
                             unsigned lo_hi)
 {
    LLVMValueRef shuffle, elems[LP_MAX_VECTOR_LENGTH];
-   unsigned i, j;
+   unsigned i;

    assert(type.length <= LP_MAX_VECTOR_LENGTH);
    assert(lo_hi < 2);

    if (type.length * type.width == 256) {
-      assert(type.length >= 4);
-      for (i = 0, j = 0; i < type.length; ++i) {
-         if (i == type.length / 4) {
-            j = type.length;
-         } else if (i == type.length / 2) {
-            j = type.length / 2;
-         } else if (i == 3 * type.length / 4) {
-            j = 3 * type.length / 4;
-         } else {
-            j += 2;
-         }
-         elems[i] = lp_build_const_int32(gallivm, j + lo_hi);
+      assert(type.length == 8);
+      assert(type.width == 32);
+      const unsigned shufvals[8] = {0, 2, 8, 10, 4, 6, 12, 14};
+      for (i = 0; i < type.length; ++i) {
+         elems[i] = lp_build_const_int32(gallivm, shufvals[i] + lo_hi);
       }
    } else {
       for (i = 0; i < type.length; ++i) {
@@ -277,7 +270,7 @@ lp_build_gather_s3tc(struct gallivm_state *gallivm,
    }
    else {
       LLVMValueRef tmp[4], cc01, cc23;
-      struct lp_type lp_type32, lp_type64, lp_type32dxt;
+      struct lp_type lp_type32, lp_type64;
       memset(&lp_type32, 0, sizeof lp_type32);
       lp_type32.width = 32;
       lp_type32.length = length;
@@ -309,10 +302,14 @@ lp_build_gather_s3tc(struct gallivm_state *gallivm,
                                               lp_build_const_extend_shuffle(gallivm, 2, 4), "");
          }
          if (length == 8) {
+            struct lp_type lp_type32_4;
+            memset(&lp_type32_4, 0, sizeof lp_type32_4);
+            lp_type32_4.width = 32;
+            lp_type32_4.length = 4;
             for (i = 0; i < 4; ++i) {
                tmp[0] = elems[i];
                tmp[1] = elems[i+4];
-               elems[i] = lp_build_concat(gallivm, tmp, lp_type32, 2);
+               elems[i] = lp_build_concat(gallivm, tmp, lp_type32_4, 2);
             }
          }
          cc01 = lp_build_interleave2_half(gallivm, lp_type32, elems[0], elems[1], 0);
@@ -811,7 +808,7 @@ s3tc_dxt3_to_rgba_aos(struct gallivm_state *gallivm,
    tmp = lp_build_select(&bld, sel_mask, alpha_low, alpha_hi);
    bit_pos = LLVMBuildAnd(builder, bit_pos,
                           lp_build_const_int_vec(gallivm, type, 0xffffffdf), "");
-   /* Warning: slow shift with per element count */
+   /* Warning: slow shift with per element count (without avx2) */
    /*
     * Could do pshufb here as well - just use appropriate 2 bits in bit_pos
     * to select the right byte with pshufb. Then for the remaining one bit
@@ -1640,7 +1637,6 @@ s3tc_decode_block_dxt5(struct gallivm_state *gallivm,
                           lp_build_const_int_vec(gallivm, type16, 8), "");
    alpha = LLVMBuildBitCast(builder, alpha,  i64t, "");
    shuffle1 = lp_build_const_shuffle1(gallivm, 0, 8);
-   /* XXX this shuffle broken with LLVM 2.8 */
    alpha0 = LLVMBuildShuffleVector(builder, alpha0, alpha0, shuffle1, "");
    alpha1 = LLVMBuildShuffleVector(builder, alpha1, alpha1, shuffle1, "");

@@ -2176,6 +2172,9 @@ lp_build_fetch_s3tc_rgba_aos(struct gallivm_state *gallivm,
       return rgba;
    }

+   /*
+    * Could use n > 8 here with avx2, but doesn't seem faster.
+    */
    if (n > 4) {
       unsigned count;
       LLVMTypeRef i8_vectype = LLVMVectorType(i8t, 4 * n);
--
2.17.1

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