Look for a loop's starting location in the llvm.loop metadata

Getting accurate locations for loops is important, because those locations are
used by the frontend to generate optimization remarks. Currently, optimization
remarks for loops often appear on the wrong line, often the first line of the
loop body instead of the loop itself. This is confusing because that line might
itself be another loop, or might be somewhere else completely if the body was
inlined function call. This happens because of the way we find the loop's
starting location. First, we look for a preheader, and if we find one, and its
terminator has a debug location, then we use that. Otherwise, we look for a
location on an instruction in the loop header.

The fallback heuristic is not bad, but will almost always find the beginning of
the body, and not the loop statement itself. The preheader location search
often fails because there's often not a preheader, and even when there is a
preheader, depending on how it was formed, it sometimes carries the location of
some preceeding code.

I don't see any good theoretical way to fix this problem. On the other hand,
this seems like a straightforward solution: Put the debug location in the
loop's llvm.loop metadata. A companion Clang patch will cause Clang to insert
llvm.loop metadata with appropriate locations when generating debugging
information. With these changes, our loop remarks have much more accurate
locations.

Differential Revision: http://reviews.llvm.org/D19738

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270771 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/LoopInfo.h

@@ -457,22 +457,7 @@ public:
   /// location by looking at the preheader and header blocks. If it
   /// cannot find a terminating instruction with location information,
   /// it returns an unknown location.
-  DebugLoc getStartLoc() const {
-    BasicBlock *HeadBB;
-
-    // Try the pre-header first.
-    if ((HeadBB = getLoopPreheader()) != nullptr)
-      if (DebugLoc DL = HeadBB->getTerminator()->getDebugLoc())
-        return DL;
-
-    // If we have no pre-header or there are no instructions with debug
-    // info in it, try the header.
-    HeadBB = getHeader();
-    if (HeadBB)
-      return HeadBB->getTerminator()->getDebugLoc();
-
-    return DebugLoc();
-  }
+  DebugLoc getStartLoc() const;
 
   StringRef getName() const {
     if (BasicBlock *Header = getHeader())

lib/Analysis/LoopInfo.cpp

@@ -22,6 +22,7 @@
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/LLVMContext.h"
@@ -298,6 +299,26 @@ bool Loop::isAnnotatedParallel() const {
   return true;
 }
 
+DebugLoc Loop::getStartLoc() const {
+  // If we have a debug location in the loop ID, then use it.
+  if (MDNode *LoopID = getLoopID())
+    for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i)
+      if (DILocation *L = dyn_cast<DILocation>(LoopID->getOperand(i)))
+        return DebugLoc(L);
+
+  // Try the pre-header first.
+  if (BasicBlock *PHeadBB = getLoopPreheader())
+    if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
+      return DL;
+
+  // If we have no pre-header or there are no instructions with debug
+  // info in it, try the header.
+  if (BasicBlock *HeadBB = getHeader())
+    return HeadBB->getTerminator()->getDebugLoc();
+
+  return DebugLoc();
+}
+
 bool Loop::hasDedicatedExits() const {
   // Each predecessor of each exit block of a normal loop is contained
   // within the loop.

test/Transforms/LoopVectorize/X86/vectorization-remarks-loopid-dbg.ll

@@ -0,0 +1,74 @@
+; RUN: opt < %s -loop-vectorize -mtriple=x86_64-unknown-linux -S -pass-remarks='loop-vectorize' 2>&1 | FileCheck -check-prefix=VECTORIZED %s
+; RUN: opt < %s -loop-vectorize -force-vector-width=1 -force-vector-interleave=4 -mtriple=x86_64-unknown-linux -S -pass-remarks='loop-vectorize' 2>&1 | FileCheck -check-prefix=UNROLLED %s
+; RUN: opt < %s -loop-vectorize -force-vector-width=1 -force-vector-interleave=1 -mtriple=x86_64-unknown-linux -S -pass-remarks-analysis='loop-vectorize' 2>&1 | FileCheck -check-prefix=NONE %s
+
+; RUN: llc < %s -mtriple x86_64-pc-linux-gnu -o - | FileCheck -check-prefix=DEBUG-OUTPUT %s
+; DEBUG-OUTPUT-NOT: .loc
+; DEBUG-OUTPUT-NOT: {{.*}}.debug_info
+
+; VECTORIZED: remark: vectorization-remarks.c:17:8: vectorized loop (vectorization width: 4, interleaved count: 1)
+; UNROLLED: remark: vectorization-remarks.c:17:8: interleaved loop (interleaved count: 4)
+; NONE: remark: vectorization-remarks.c:17:8: loop not vectorized: vectorization and interleaving are explicitly disabled, or vectorize width and interleave count are both set to 1
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define i32 @foo(i32 %n) #0 !dbg !4 {
+entry:
+  %diff = alloca i32, align 4
+  %cb = alloca [16 x i8], align 16
+  %cc = alloca [16 x i8], align 16
+  store i32 0, i32* %diff, align 4, !tbaa !11
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %add8 = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  %arrayidx = getelementptr inbounds [16 x i8], [16 x i8]* %cb, i64 0, i64 %indvars.iv
+  %0 = load i8, i8* %arrayidx, align 1, !tbaa !21
+  %conv = sext i8 %0 to i32
+  %arrayidx2 = getelementptr inbounds [16 x i8], [16 x i8]* %cc, i64 0, i64 %indvars.iv
+  %1 = load i8, i8* %arrayidx2, align 1, !tbaa !21
+  %conv3 = sext i8 %1 to i32
+  %sub = sub i32 %conv, %conv3
+  %add = add nsw i32 %sub, %add8
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 16
+  br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !25
+
+for.end:                                          ; preds = %for.body
+  store i32 %add, i32* %diff, align 4, !tbaa !11
+  call void @ibar(i32* %diff) #2
+  ret i32 0
+}
+
+declare void @ibar(i32*) #1
+
+!llvm.module.flags = !{!7, !8}
+!llvm.ident = !{!9}
+!llvm.dbg.cu = !{!24}
+
+!1 = !DIFile(filename: "vectorization-remarks.c", directory: ".")
+!2 = !{}
+!3 = !{!4}
+!4 = distinct !DISubprogram(name: "foo", line: 5, isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: true, unit: !24, scopeLine: 6, file: !1, scope: !5, type: !6, variables: !2)
+!5 = !DIFile(filename: "vectorization-remarks.c", directory: ".")
+!6 = !DISubroutineType(types: !2)
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 1, !"Debug Info Version", i32 3}
+!9 = !{!"clang version 3.5.0 "}
+!10 = !DILocation(line: 8, column: 3, scope: !4)
+!11 = !{!12, !12, i64 0}
+!12 = !{!"int", !13, i64 0}
+!13 = !{!"omnipotent char", !14, i64 0}
+!14 = !{!"Simple C/C++ TBAA"}
+!15 = !DILocation(line: 17, column: 8, scope: !16)
+!16 = distinct !DILexicalBlock(line: 17, column: 8, file: !1, scope: !17)
+!17 = distinct !DILexicalBlock(line: 17, column: 8, file: !1, scope: !18)
+!18 = distinct !DILexicalBlock(line: 17, column: 3, file: !1, scope: !4)
+!19 = !DILocation(line: 18, column: 5, scope: !20)
+!20 = distinct !DILexicalBlock(line: 17, column: 27, file: !1, scope: !18)
+!21 = !{!13, !13, i64 0}
+!22 = !DILocation(line: 20, column: 3, scope: !4)
+!23 = !DILocation(line: 21, column: 3, scope: !4)
+!24 = distinct !DICompileUnit(language: DW_LANG_C89, file: !1, emissionKind: NoDebug)
+!25 = !{!25, !15}