glsl: Correctly handle function matching when there are multiple inexact matches

This is a squash cherry pick commit of:

    glsl: Find the "closest" signature when there are multiple matches.

    Previously, ir_function::matching_signature had a fatal bug: if a
    function had more than one non-exact match, it would simply return NULL.

    This occured, for example, when looking for max(uvec3, uvec3):
    - max(vec3, vec3)   -> score 1 (found first)
    - max(ivec3, ivec3) -> score 1 (found second...used to return NULL here)
    - max(uvec3, uvec3) -> score 0 (exact match...the right answer)

    This did not occur for max(ivec3, ivec3) since the second match found
    was an exact match.

    The new behavior is to return a match with the lowest score.  If there
    is an exact match, that will be returned.  Otherwise, a match with the
    least number of implicit conversions is chosen.

    Fixes piglit tests max-uvec3.vert and glsl-inexact-overloads.shader_test.

    NOTE: This is a candidate for the 7.10 and 7.11 branches.

    Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
    Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
    Reviewed-by: Eric Anholt <eric@anholt.net>
    (cherry picked from commit 60eb63a855)

    glsl: Suppress warning from matching_signature change.

    gcc isn't smart enough to see that we only look at matched_score after
    we've initialized it (because match != NULL happens at the same time)
    (cherry picked from commit b043409adf)

    glsl: Reject ambiguous function calls (multiple inexact matches).

    According to the GLSL 1.20 specification, "it is a semantic error if
    there are multiple ways to apply [implicit] conversions [...] such that
    the call can be made to match multiple signatures."

    Fixes a regression caused by 60eb63a855,
    which implemented the wrong policy of finding a "closest" match.
    However, this is not a revert, since the original code failed to
    continue looking for an exact match once it found two inexact matches.

    It's OK to have multiple inexact matches if there's also an exact match.

    NOTE: This is a candidate for the 7.10 and 7.11 branches.

    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38971
    Reviewed-by: Eric Anholt <eric@anholt.net>
    Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
    (cherry picked from commit 7304909d65)

src/glsl/ir_function.cpp

@@ -165,7 +165,18 @@ ir_function_signature *
 ir_function::matching_signature(const exec_list *actual_parameters)
 {
    ir_function_signature *match = NULL;
+   bool multiple_inexact_matches = false;
 
+   /* From page 42 (page 49 of the PDF) of the GLSL 1.20 spec:
+    *
+    * "If an exact match is found, the other signatures are ignored, and
+    *  the exact match is used.  Otherwise, if no exact match is found, then
+    *  the implicit conversions in Section 4.1.10 "Implicit Conversions" will
+    *  be applied to the calling arguments if this can make their types match
+    *  a signature.  In this case, it is a semantic error if there are
+    *  multiple ways to apply these conversions to the actual arguments of a
+    *  call such that the call can be made to match multiple signatures."
+    */
    foreach_iter(exec_list_iterator, iter, signatures) {
       ir_function_signature *const sig =
 	 (ir_function_signature *) iter.get();
@@ -173,17 +184,28 @@ ir_function::matching_signature(const exec_list *actual_parameters)
       const int score = parameter_lists_match(& sig->parameters,
 					      actual_parameters);
 
+      /* If we found an exact match, simply return it */
       if (score == 0)
 	 return sig;
 
       if (score > 0) {
-	 if (match != NULL)
-	    return NULL;
-
-	 match = sig;
+	 if (match == NULL)
+	    match = sig;
+	 else
+	    multiple_inexact_matches = true;
       }
    }
 
+   /* There is no exact match (we would have returned it by now).  If there
+    * are multiple inexact matches, the call is ambiguous, which is an error.
+    *
+    * FINISHME: Report a decent error.  Returning NULL will likely result in
+    * FINISHME: a "no matching signature" error; it should report that the
+    * FINISHME: call is ambiguous.  But reporting errors from here is hard.
+    */
+   if (multiple_inexact_matches)
+      return NULL;
+
    return match;
 }