Index: llvm/trunk/include/llvm/MC/MCStreamer.h
===================================================================
--- llvm/trunk/include/llvm/MC/MCStreamer.h
+++ llvm/trunk/include/llvm/MC/MCStreamer.h
@@ -870,7 +870,7 @@
 
   virtual MCSymbol *getDwarfLineTableSymbol(unsigned CUID);
   virtual void EmitCFISections(bool EH, bool Debug);
-  void EmitCFIStartProc(bool IsSimple);
+  void EmitCFIStartProc(bool IsSimple, SMLoc Loc = SMLoc());
   void EmitCFIEndProc();
   virtual void EmitCFIDefCfa(int64_t Register, int64_t Offset);
   virtual void EmitCFIDefCfaOffset(int64_t Offset);
Index: llvm/trunk/lib/MC/MCParser/AsmParser.cpp
===================================================================
--- llvm/trunk/lib/MC/MCParser/AsmParser.cpp
+++ llvm/trunk/lib/MC/MCParser/AsmParser.cpp
@@ -3919,8 +3919,13 @@
         parseToken(AsmToken::EndOfStatement))
       return addErrorSuffix(" in '.cfi_startproc' directive");
   }
-
-  getStreamer().EmitCFIStartProc(!Simple.empty());
+  
+  // TODO(kristina): Deal with a corner case of incorrect diagnostic context
+  // being produced if this directive is emitted as part of preprocessor macro
+  // expansion which can *ONLY* happen if Clang's cc1as is the API consumer.
+  // Tools like llvm-mc on the other hand are not affected by it, and report
+  // correct context information.
+  getStreamer().EmitCFIStartProc(!Simple.empty(), Lexer.getLoc());
   return false;
 }
 
Index: llvm/trunk/lib/MC/MCStreamer.cpp
===================================================================
--- llvm/trunk/lib/MC/MCStreamer.cpp
+++ llvm/trunk/lib/MC/MCStreamer.cpp
@@ -347,10 +347,10 @@
   assert(EH || Debug);
 }
 
-void MCStreamer::EmitCFIStartProc(bool IsSimple) {
+void MCStreamer::EmitCFIStartProc(bool IsSimple, SMLoc Loc) {
   if (hasUnfinishedDwarfFrameInfo())
-    getContext().reportError(
-        SMLoc(), "starting new .cfi frame before finishing the previous one");
+    return getContext().reportError(
+        Loc, "starting new .cfi frame before finishing the previous one");
 
   MCDwarfFrameInfo Frame;
   Frame.IsSimple = IsSimple;
Index: llvm/trunk/test/MC/X86/cfi-open-within-another-crash.s
===================================================================
--- llvm/trunk/test/MC/X86/cfi-open-within-another-crash.s
+++ llvm/trunk/test/MC/X86/cfi-open-within-another-crash.s
@@ -0,0 +1,18 @@
+# Test for D51695 ensuring there is no crash when two .cfi_startproc are opened
+# without the first one being closed.
+
+# RUN: not llvm-mc %s -filetype=obj -triple=x86_64-unknown-linux -o /dev/null 2>&1 | FileCheck %s
+
+.text
+.globl proc_one
+proc_one:
+ .cfi_startproc
+ 
+.text
+.globl proc_two
+proc_two:
+ .cfi_startproc
+ 
+ .cfi_endproc
+
+# CHECK: error: starting new .cfi frame before finishing the previous one
Index: llvm/trunk/test/MC/X86/cfi-scope-errors.s
===================================================================
--- llvm/trunk/test/MC/X86/cfi-scope-errors.s
+++ llvm/trunk/test/MC/X86/cfi-scope-errors.s
@@ -1,6 +1,5 @@
-# RUN: not llvm-mc %s -triple x86_64-linux -o /dev/null 2>&1 | FileCheck %s --implicit-check-not=error:
-
-# FIXME: Push source locations into diagnostics.
+# RUN: not llvm-mc %s -triple x86_64-linux -o /dev/null 2>&1 | FileCheck %s
+# RUN: not llvm-mc %s -triple x86_64-linux -filetype=obj -o /dev/null 2>&1 | FileCheck %s
 
 .text
 .cfi_def_cfa rsp, 8
@@ -9,8 +8,16 @@
 .cfi_startproc
 nop
 
+# TODO(kristina): As Reid suggested, this now supports source locations as a side effect
+# of another patch aimed at fixing the crash that would occur here, however the other
+# ones do not unfortunately. Will address it in a further patch propogating SMLoc down to
+# other CFI directives at which point more LINE checks can be added to ensure proper source
+# location reporting.
+
+# This tests source location correctness as well as the error and it not crashing.
+# CHECK: [[@LINE+2]]:1: error: starting new .cfi frame before finishing the previous one
 .cfi_startproc
-# CHECK: error: starting new .cfi frame before finishing the previous one
+
 nop
 .cfi_endproc