[Mesa-dev] [PATCH] gallivm: fix SCALED -> NORM conversions

[sroland at vmware.com]

From: Roland Scheidegger <sroland at vmware.com>

Such conversions (which are most likely rather pointless in practice) were
resulting in shifts with negative shift counts and shifts with counts the same
as the bit width. This was always undefined in llvm, the code generated was
rather horrendous but happened to work.
So make sure such shifts are filtered out and replaced with something that
works (the generated code is still just as horrendous as before).

This fixes lp_test_format, https://bugs.freedesktop.org/show_bug.cgi?id=73846.
---
 src/gallium/auxiliary/gallivm/lp_bld_conv.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/src/gallium/auxiliary/gallivm/lp_bld_conv.c b/src/gallium/auxiliary/gallivm/lp_bld_conv.c
index d3bf621..fe8f08b 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_conv.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_conv.c
@@ -794,7 +794,8 @@ lp_build_conv(struct gallivm_state *gallivm,
       unsigned dst_offset = lp_const_offset(dst_type);
 
       /* Compensate for different offsets */
-      if (dst_offset > src_offset && src_type.width > dst_type.width) {
+      /* sscaled -> unorm and similar would cause negative shift count, skip */
+      if (dst_offset > src_offset && src_type.width > dst_type.width && src_shift > 0) {
          for (i = 0; i < num_tmps; ++i) {
             LLVMValueRef shifted;
             LLVMValueRef shift = lp_build_const_int_vec(gallivm, tmp_type, src_shift - 1);
@@ -903,11 +904,24 @@ lp_build_conv(struct gallivm_state *gallivm,
 
        if (src_shift < dst_shift) {
           LLVMValueRef pre_shift[LP_MAX_VECTOR_LENGTH];
-          LLVMValueRef shift = lp_build_const_int_vec(gallivm, tmp_type, dst_shift - src_shift);
 
-          for (i = 0; i < num_tmps; ++i) {
-             pre_shift[i] = tmp[i];
-             tmp[i] = LLVMBuildShl(builder, tmp[i], shift, "");
+          if (dst_shift - src_shift < dst_type.width) {
+             LLVMValueRef shift = lp_build_const_int_vec(gallivm, tmp_type,
+                                                         dst_shift - src_shift);
+             for (i = 0; i < num_tmps; ++i) {
+                pre_shift[i] = tmp[i];
+                tmp[i] = LLVMBuildShl(builder, tmp[i], shift, "");
+             }
+          }
+          else {
+             /*
+              * This happens for things like sscaled -> unorm conversions.
+              * Negative shift counts cause undefined results, so hack around it.
+              */
+             for (i = 0; i < num_tmps; ++i) {
+                pre_shift[i] = tmp[i];
+                tmp[i] = lp_build_zero(gallivm, dst_type);
+             }
           }
 
           /* Compensate for different offsets */
-- 
1.9.1