Index: lib/Transforms/Scalar/SampleProfile.cpp
===================================================================
--- lib/Transforms/Scalar/SampleProfile.cpp
+++ lib/Transforms/Scalar/SampleProfile.cpp
@@ -644,6 +644,9 @@
 unsigned SampleProfileLoader::getFunctionLoc(Function &F) {
   NamedMDNode *CUNodes = F.getParent()->getNamedMetadata("llvm.dbg.cu");
   if (CUNodes) {
+    // If the file has been compiled with debug information enabled, we
+    // can find \p F by traversing the compile units in the llvm.dbg.cu
+    // annotation.
     for (unsigned I = 0, E1 = CUNodes->getNumOperands(); I != E1; ++I) {
       DICompileUnit CU(CUNodes->getOperand(I));
       DIArray Subprograms = CU.getSubprograms();
@@ -653,10 +656,29 @@
           return Subprogram.getLineNumber();
       }
     }
+  } else {
+    // Otherwise, we look for the first instruction that has a debug annotation
+    // leading back to \p F.
+    for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+      BasicBlock *B = I;
+      for (BasicBlock::iterator BI = B->begin(), BE = B->end(); BI != BE;
+           ++BI) {
+        Instruction &Inst = *BI;
+        DebugLoc DLoc = Inst.getDebugLoc();
+        const MDNode *Scope = DLoc.getScopeNode(*Ctx);
+        DISubprogram Subprogram = getDISubprogram(Scope);
+        if (Subprogram.describes(&F))
+          return Subprogram.getLineNumber();
+      }
+    }
   }
 
+  // If could not find the start of \p F, emit a diagnostic to warn the user
+  // about the missed opportunity.
   F.getContext().diagnose(DiagnosticInfoSampleProfile(
-      "No debug information found in function " + F.getName()));
+      "No debug information found in function " + F.getName() +
+          ": Function profile not used",
+      DS_Warning));
   return 0;
 }
 
Index: test/Transforms/SampleProfile/loc-tracking-only.ll
===================================================================
--- /dev/null
+++ test/Transforms/SampleProfile/loc-tracking-only.ll
@@ -0,0 +1,103 @@
+; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/calls.prof | opt -analyze -branch-prob | FileCheck %s
+
+; This test is identical to calls.ll but it is missing the llvm.dbg.cu
+; annotation. This emulates the effect of the user having only used
+; -fprofile-sample-use without -gmlt when invoking the driver.
+
+@.str = private unnamed_addr constant [11 x i8] c"sum is %d\0A\00", align 1
+
+; Function Attrs: nounwind uwtable
+define i32 @_Z3sumii(i32 %x, i32 %y) {
+entry:
+  %x.addr = alloca i32, align 4
+  %y.addr = alloca i32, align 4
+  store i32 %x, i32* %x.addr, align 4
+  store i32 %y, i32* %y.addr, align 4
+  %0 = load i32* %x.addr, align 4, !dbg !11
+  %1 = load i32* %y.addr, align 4, !dbg !11
+  %add = add nsw i32 %0, %1, !dbg !11
+  ret i32 %add, !dbg !11
+}
+
+; Function Attrs: uwtable
+define i32 @main() {
+entry:
+  %retval = alloca i32, align 4
+  %s = alloca i32, align 4
+  %i = alloca i32, align 4
+  store i32 0, i32* %retval
+  store i32 0, i32* %i, align 4, !dbg !12
+  br label %while.cond, !dbg !13
+
+while.cond:                                       ; preds = %if.end, %entry
+  %0 = load i32* %i, align 4, !dbg !14
+  %inc = add nsw i32 %0, 1, !dbg !14
+  store i32 %inc, i32* %i, align 4, !dbg !14
+  %cmp = icmp slt i32 %0, 400000000, !dbg !14
+  br i1 %cmp, label %while.body, label %while.end, !dbg !14
+; CHECK: edge while.cond -> while.body probability is 5391 / 5392 = 99.9815% [HOT edge]
+; CHECK: edge while.cond -> while.end probability is 1 / 5392 = 0.018546%
+
+while.body:                                       ; preds = %while.cond
+  %1 = load i32* %i, align 4, !dbg !16
+  %cmp1 = icmp ne i32 %1, 100, !dbg !16
+  br i1 %cmp1, label %if.then, label %if.else, !dbg !16
+; Without discriminator information, the profiler used to think that
+; both branches out of while.body had the same weight. In reality,
+; the edge while.body->if.then is taken most of the time.
+;
+; CHECK: edge while.body -> if.then probability is 5752 / 5753 = 99.9826% [HOT edge]
+; CHECK: edge while.body -> if.else probability is 1 / 5753 = 0.0173822%
+
+
+if.then:                                          ; preds = %while.body
+  %2 = load i32* %i, align 4, !dbg !18
+  %3 = load i32* %s, align 4, !dbg !18
+  %call = call i32 @_Z3sumii(i32 %2, i32 %3), !dbg !18
+  store i32 %call, i32* %s, align 4, !dbg !18
+  br label %if.end, !dbg !18
+
+if.else:                                          ; preds = %while.body
+  store i32 30, i32* %s, align 4, !dbg !20
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  br label %while.cond, !dbg !22
+
+while.end:                                        ; preds = %while.cond
+  %4 = load i32* %s, align 4, !dbg !24
+  %call2 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([11 x i8]* @.str, i32 0, i32 0), i32 %4), !dbg !24
+  ret i32 0, !dbg !25
+}
+
+declare i32 @printf(i8*, ...) #2
+
+!llvm.module.flags = !{!8, !9}
+!llvm.ident = !{!10}
+
+!0 = metadata !{metadata !"0x11\004\00clang version 3.5 \000\00\000\00\000", metadata !1, metadata !2, metadata !2, metadata !3, metadata !2, metadata !2} ; [ DW_TAG_compile_unit ] [./calls.cc] [DW_LANG_C_plus_plus]
+!1 = metadata !{metadata !"calls.cc", metadata !"."}
+!2 = metadata !{}
+!3 = metadata !{metadata !4, metadata !7}
+!4 = metadata !{metadata !"0x2e\00sum\00sum\00\003\000\001\000\006\00256\000\003", metadata !1, metadata !5, metadata !6, null, i32 (i32, i32)* @_Z3sumii, null, null, metadata !2} ; [ DW_TAG_subprogram ] [line 3] [def] [sum]
+!5 = metadata !{metadata !"0x29", metadata !1}          ; [ DW_TAG_file_type ] [./calls.cc]
+!6 = metadata !{metadata !"0x15\00\000\000\000\000\000\000", i32 0, null, null, metadata !2, null, null, null} ; [ DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset 0] [from ]
+!7 = metadata !{metadata !"0x2e\00main\00main\00\007\000\001\000\006\00256\000\007", metadata !1, metadata !5, metadata !6, null, i32 ()* @main, null, null, metadata !2} ; [ DW_TAG_subprogram ] [line 7] [def] [main]
+!8 = metadata !{i32 2, metadata !"Dwarf Version", i32 4}
+!9 = metadata !{i32 1, metadata !"Debug Info Version", i32 2}
+!10 = metadata !{metadata !"clang version 3.5 "}
+!11 = metadata !{i32 4, i32 0, metadata !4, null}
+!12 = metadata !{i32 8, i32 0, metadata !7, null}
+!13 = metadata !{i32 9, i32 0, metadata !7, null}
+!14 = metadata !{i32 9, i32 0, metadata !15, null}
+!15 = metadata !{metadata !"0xb\001", metadata !1, metadata !7} ; [ DW_TAG_lexical_block ] [./calls.cc]
+!16 = metadata !{i32 10, i32 0, metadata !17, null}
+!17 = metadata !{metadata !"0xb\0010\000\000", metadata !1, metadata !7} ; [ DW_TAG_lexical_block ] [./calls.cc]
+!18 = metadata !{i32 10, i32 0, metadata !19, null}
+!19 = metadata !{metadata !"0xb\001", metadata !1, metadata !17} ; [ DW_TAG_lexical_block ] [./calls.cc]
+!20 = metadata !{i32 10, i32 0, metadata !21, null}
+!21 = metadata !{metadata !"0xb\002", metadata !1, metadata !17} ; [ DW_TAG_lexical_block ] [./calls.cc]
+!22 = metadata !{i32 10, i32 0, metadata !23, null}
+!23 = metadata !{metadata !"0xb\003", metadata !1, metadata !17} ; [ DW_TAG_lexical_block ] [./calls.cc]
+!24 = metadata !{i32 11, i32 0, metadata !7, null}
+!25 = metadata !{i32 12, i32 0, metadata !7, null}
Index: test/Transforms/SampleProfile/syntax.ll
===================================================================
--- test/Transforms/SampleProfile/syntax.ll
+++ test/Transforms/SampleProfile/syntax.ll
@@ -1,4 +1,4 @@
-; RUN: not opt < %s -sample-profile -sample-profile-file=%S/Inputs/syntax.prof 2>&1 | FileCheck -check-prefix=NO-DEBUG %s
+; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/syntax.prof 2>&1 | FileCheck -check-prefix=NO-DEBUG %s
 ; RUN: not opt < %s -sample-profile -sample-profile-file=missing.prof 2>&1 | FileCheck -check-prefix=MISSING-FILE %s
 ; RUN: not opt < %s -sample-profile -sample-profile-file=%S/Inputs/bad_fn_header.prof 2>&1 | FileCheck -check-prefix=BAD-FN-HEADER %s
 ; RUN: not opt < %s -sample-profile -sample-profile-file=%S/Inputs/bad_sample_line.prof 2>&1 | FileCheck -check-prefix=BAD-SAMPLE-LINE %s
@@ -11,7 +11,7 @@
 entry:
   ret void
 }
-; NO-DEBUG: error: No debug information found in function empty
+; NO-DEBUG: warning: No debug information found in function empty: Function profile not used
 ; MISSING-FILE: error: missing.prof:
 ; BAD-FN-HEADER: error: {{.*}}bad_fn_header.prof:1: Expected 'mangled_name:NUM:NUM', found 3empty:100:BAD
 ; BAD-SAMPLE-LINE: error: {{.*}}bad_sample_line.prof:3: Expected 'NUM[.NUM]: NUM[ mangled_name:NUM]*', found 1: BAD