Block pretty printing (partial re-revert of 1177123002).

src/compiler/code-generator.cc

Index: src/compiler/code-generator.cc
diff --git a/src/compiler/code-generator.cc b/src/compiler/code-generator.cc
index 374e6774305dfa437109146a0fec0972ec069172..59d1000113a8929545f77560554e31348fc1ea1e 100644
--- a/src/compiler/code-generator.cc
+++ b/src/compiler/code-generator.cc
@@ -97,12 +97,29 @@ Handle<Code> CodeGenerator::GenerateCode() {
       if (FLAG_code_comments) {
         // TODO(titzer): these code comments are a giant memory leak.
         Vector<char> buffer = Vector<char>::New(200);
-        SNPrintF(buffer, "-- B%d start%s%s%s%s --", block->rpo_number().ToInt(),
-                 block->IsDeferred() ? " (deferred)" : "",
-                 block->needs_frame() ? "" : " (no frame)",
-                 block->must_construct_frame() ? " (construct frame)" : "",
-                 block->must_deconstruct_frame() ? " (deconstruct frame)" : "");
-        masm()->RecordComment(buffer.start());
+        char* buffer_start = buffer.start();
+
+        int next = SNPrintF(
+            buffer, "-- B%d start%s%s%s%s", block->rpo_number().ToInt(),
+            block->IsDeferred() ? " (deferred)" : "",
+            block->needs_frame() ? "" : " (no frame)",
+            block->must_construct_frame() ? " (construct frame)" : "",
+            block->must_deconstruct_frame() ? " (deconstruct frame)" : "");
+
+        buffer = buffer.SubVector(next, buffer.length());
+
+        if (block->IsLoopHeader()) {
+          next =
+              SNPrintF(buffer, " (loop up to %d)", block->loop_end().ToInt());
+          buffer = buffer.SubVector(next, buffer.length());
+        }
+        if (block->loop_header().IsValid()) {
+          next =
+              SNPrintF(buffer, " (in loop %d)", block->loop_header().ToInt());
+          buffer = buffer.SubVector(next, buffer.length());
+        }
+        SNPrintF(buffer, " --");
+        masm()->RecordComment(buffer_start);
       }
       masm()->bind(GetLabel(current_block_));
       for (int i = block->code_start(); i < block->code_end(); ++i) {