Subzero: Add branch optimization.

src/IceInstX8632.cpp

Index: src/IceInstX8632.cpp
diff --git a/src/IceInstX8632.cpp b/src/IceInstX8632.cpp
index a61805e2d96b53ef73566c28fed2224a20b69de4..9c8053d22744996f3ac1104d68ed1e78b4070576 100644
--- a/src/IceInstX8632.cpp
+++ b/src/IceInstX8632.cpp
@@ -24,11 +24,12 @@ namespace Ice {
 namespace {
 
 const struct InstX8632BrAttributes_ {
+  InstX8632::BrCond Opposite;
   const char *DisplayString;
   const char *EmitString;
 } InstX8632BrAttributes[] = {
-#define X(tag, dump, emit)                                                     \
-  { dump, emit }                                                               \
+#define X(tag, opp, dump, emit)                                                \
+  { InstX8632::opp, dump, emit }                                               \
   ,
     ICEINSTX8632BR_TABLE
 #undef X
@@ -128,8 +129,10 @@ IceString InstX8632Label::getName(const Cfg *Func) const {
   return ".L" + Func->getFunctionName() + "$local$__" + buf;
 }
 
-InstX8632Br::InstX8632Br(Cfg *Func, CfgNode *TargetTrue, CfgNode *TargetFalse,
-                         InstX8632Label *Label, InstX8632::BrCond Condition)
+InstX8632Br::InstX8632Br(Cfg *Func, const CfgNode *TargetTrue,
+                         const CfgNode *TargetFalse,
+                         const InstX8632Label *Label,
+                         InstX8632::BrCond Condition)
     : InstX8632(Func, InstX8632::Br, 0, NULL), Condition(Condition),
       TargetTrue(TargetTrue), TargetFalse(TargetFalse), Label(Label) {}
 
@@ -301,6 +304,37 @@ void InstX8632Label::dump(const Cfg *Func) const {
   Str << getName(Func) << ":";
 }
 
+void InstX8632Br::optimizeBranch(const CfgNode *NextNode) {
+  // If there is no next block, then there is no fallthrough to
+  // optimize.
+  if (NextNode == NULL)
+    return;
+  // Intra-block conditional branches can't be optimized.
+  if (Label)
+    return;
+  // Unconditional branch to the next node can be removed.
+  if (Condition == Br_None && getTargetFalse() == NextNode) {
+    setDeleted();
+    return;
+  }
+  // If the fallthrough is to the next node, set fallthrough to NULL
+  // to indicate.
+  if (getTargetFalse() == NextNode) {
+    TargetFalse = NULL;
+    return;
+  }
+  // If TargetTrue is the next node, and TargetFalse is non-NULL, then
+  // invert the branch condition, swap the targets, and set new
+  // fallthrough to NULL.
+  if (getTargetTrue() == NextNode && getTargetFalse()) {
+    assert(Condition != Br_None);
+    Condition = InstX8632BrAttributes[Condition].Opposite;
+    TargetTrue = getTargetFalse();
+    TargetFalse = NULL;
+    return;
+  }
+}
+
 void InstX8632Br::emit(const Cfg *Func) const {
   Ostream &Str = Func->getContext()->getStrEmit();
   Str << "\t";