llvmpipe: Make lerping work for 8.8 fixed point values.

diff --git a/src/gallium/drivers/llvmpipe/lp_bld_arit.c b/src/gallium/drivers/llvmpipe/lp_bld_arit.c
index 83ca06a..93e797c 100644 (file)
--- a/src/gallium/drivers/llvmpipe/lp_bld_arit.c
+++ b/src/gallium/drivers/llvmpipe/lp_bld_arit.c
@@ -361,6 +361,8 @@ lp_build_mul(struct lp_build_context *bld,
              LLVMValueRef b)
 {
    const struct lp_type type = bld->type;
+   LLVMValueRef shift;
+   LLVMValueRef res;
 
    if(a == bld->zero)
       return bld->zero;
@@ -394,10 +396,31 @@ lp_build_mul(struct lp_build_context *bld,
       assert(0);
    }
 
-   if(LLVMIsConstant(a) && LLVMIsConstant(b))
-      return LLVMConstMul(a, b);
+   if(type.fixed)
+      shift = lp_build_int_const_scalar(type, type.width/2);
+   else
+      shift = NULL;
+
+   if(LLVMIsConstant(a) && LLVMIsConstant(b)) {
+      res =  LLVMConstMul(a, b);
+      if(shift) {
+         if(type.sign)
+            res = LLVMConstAShr(res, shift);
+         else
+            res = LLVMConstLShr(res, shift);
+      }
+   }
+   else {
+      res = LLVMBuildMul(bld->builder, a, b, "");
+      if(shift) {
+         if(type.sign)
+            res = LLVMBuildAShr(bld->builder, res, shift, "");
+         else
+            res = LLVMBuildLShr(bld->builder, res, shift, "");
+      }
+   }
 
-   return LLVMBuildMul(bld->builder, a, b, "");
+   return res;
 }
 
 
@@ -432,13 +455,36 @@ lp_build_div(struct lp_build_context *bld,
 }
 
 
+/**
+ * Linear interpolation.
+ *
+ * This also works for integer values with a few caveats.
+ *
+ * @sa http://www.stereopsis.com/doubleblend.html
+ */
 LLVMValueRef
 lp_build_lerp(struct lp_build_context *bld,
               LLVMValueRef x,
               LLVMValueRef v0,
               LLVMValueRef v1)
 {
-   return lp_build_add(bld, v0, lp_build_mul(bld, x, lp_build_sub(bld, v1, v0)));
+   LLVMValueRef delta;
+   LLVMValueRef res;
+
+   delta = lp_build_sub(bld, v1, v0);
+
+   res = lp_build_mul(bld, x, delta);
+
+   res = lp_build_add(bld, v0, res);
+
+   if(bld->type.fixed)
+      /* XXX: This step is necessary for lerping 8bit colors stored on 16bits,
+       * but it will be wrong for other uses. Basically we need a more
+       * powerful lp_type, capable of further distinguishing the values
+       * interpretation from the value storage. */
+      res = LLVMBuildAnd(bld->builder, res, lp_build_int_const_scalar(bld->type, (1 << bld->type.width/2) - 1), "");
+
+   return res;
 }