Enable tessellating GPU path renderer.

This also contains a fix to remove recursion from the sorted_merge()
step. This was essentially tail-recursion, and was causing stack
exhaustion on some platforms. Making it iterative fixes the issue.

Note: this CL will affect a large number of GPU GM results.

R=bsalomon@google.com
BUG=

Review URL: https://codereview.chromium.org/1080113004

2 files changed, 28 insertions, 16 deletions

diff --git a/src/gpu/GrAddPathRenderers_default.cpp b/src/gpu/GrAddPathRenderers_default.cpp
index f5d69348f3..e60d536720 100644
--- a/src/gpu/GrAddPathRenderers_default.cpp
+++ b/src/gpu/GrAddPathRenderers_default.cpp
@@ -20,7 +20,7 @@
 #endif
 
 #ifndef GR_TESSELLATING_PATH_RENDERING
-#define GR_TESSELLATING_PATH_RENDERING 0
+#define GR_TESSELLATING_PATH_RENDERING 1
 #endif
 
 void GrPathRenderer::AddPathRenderers(GrContext* ctx, GrPathRendererChain* chain) {
diff --git a/src/gpu/GrTessellatingPathRenderer.cpp b/src/gpu/GrTessellatingPathRenderer.cpp
index e9e8da240b..14176db4ec 100644
--- a/src/gpu/GrTessellatingPathRenderer.cpp
+++ b/src/gpu/GrTessellatingPathRenderer.cpp
@@ -1072,24 +1072,36 @@ void merge_sort(Vertex** head) {
     *head = sorted_merge(a, b);
 }
 
-Vertex* sorted_merge(Vertex* a, Vertex* b) {
-    if (!a) {
-        return b;
-    } else if (!b) {
-        return a;
-    }
+inline void append_vertex(Vertex* v, Vertex** head, Vertex** tail) {
+    insert<Vertex, &Vertex::fPrev, &Vertex::fNext>(v, *tail, NULL, head, tail);
+}
 
-    Vertex* result = NULL;
+inline void append_vertex_list(Vertex* v, Vertex** head, Vertex** tail) {
+    insert<Vertex, &Vertex::fPrev, &Vertex::fNext>(v, *tail, v->fNext, head, tail);
+}
 
-    if (sweep_lt(a->fPoint, b->fPoint)) {
-        result = a;
-        result->fNext = sorted_merge(a->fNext, b);
-    } else {
-        result = b;
-        result->fNext = sorted_merge(a, b->fNext);
+Vertex* sorted_merge(Vertex* a, Vertex* b) {
+    Vertex* head = NULL;
+    Vertex* tail = NULL;
+
+    while (a && b) {
+        if (sweep_lt(a->fPoint, b->fPoint)) {
+            Vertex* next = a->fNext;
+            append_vertex(a, &head, &tail);
+            a = next;
+        } else {
+            Vertex* next = b->fNext;
+            append_vertex(b, &head, &tail);
+            b = next;
+        }
+    }
+    if (a) {
+        append_vertex_list(a, &head, &tail);
+    }
+    if (b) {
+        append_vertex_list(b, &head, &tail);
     }
-    result->fNext->fPrev = result;
-    return result;
+    return head;
 }
 
 // Stage 4: Simplify the mesh by inserting new vertices at intersecting edges.