Make MachineBasicBlock::updateTerminator to update DebugLoc as well

Summary:
Currently MachineBasicBlock::updateTerminator simply drops DebugLoc for newly created branch instructions, which may cause incorrect stepping and/or imprecise sample profile data. Below is an example:

```
  1 extern int bar(int x);
  2
  3 int foo(int *begin, int *end) {
  4   int *i;
  5   int ret = 0;
  6   for (
  7       i = begin ;
  8       i != end ;
  9       i++)
 10   {
 11       ret += bar(*i);
 12   }
 13   return ret;
 14 }
```

Below is a bitcode of 'foo' at the end of LLVM-IR level optimizations with -O3:

```
define i32 @foo(i32* readonly %begin, i32* readnone %end) !dbg !4 {
entry:
  %cmp6 = icmp eq i32* %begin, %end, !dbg !9
  br i1 %cmp6, label %for.end, label %for.body.preheader, !dbg !12

for.body.preheader:                               ; preds = %entry
  br label %for.body, !dbg !13

for.body:                                         ; preds = %for.body.preheader, %for.body
  %ret.08 = phi i32 [ %add, %for.body ], [ 0, %for.body.preheader ]
  %i.07 = phi i32* [ %incdec.ptr, %for.body ], [ %begin, %for.body.preheader ]
  %0 = load i32, i32* %i.07, align 4, !dbg !13, !tbaa !15
  %call = tail call i32 @bar(i32 %0), !dbg !19
  %add = add nsw i32 %call, %ret.08, !dbg !20
  %incdec.ptr = getelementptr inbounds i32, i32* %i.07, i64 1, !dbg !21
  %cmp = icmp eq i32* %incdec.ptr, %end, !dbg !9
  br i1 %cmp, label %for.end.loopexit, label %for.body, !dbg !12, !llvm.loop !22

for.end.loopexit:                                 ; preds = %for.body
  br label %for.end, !dbg !24

for.end:                                          ; preds = %for.end.loopexit, %entry
  %ret.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.end.loopexit ]
  ret i32 %ret.0.lcssa, !dbg !24
}
```

where

```
!12 = !DILocation(line: 6, column: 3, scope: !11)
```

. As you can see, the terminator of 'entry' block, which is a loop control branch, has a DebugLoc of line 6, column 3. Howerver, after the execution of 'MachineBlock::updateTerminator' function, which is triggered by MachineSinking pass, the DebugLoc info is dropped as below (see there's no debug-location for JNE_1):

```
  bb.0.entry:
    successors: %bb.4(0x30000000), %bb.1.for.body.preheader(0x50000000)
    liveins: %rdi, %rsi

    %6 = COPY %rsi
    %5 = COPY %rdi
    %8 = SUB64rr %5, %6, implicit-def %eflags, debug-location !9
    JNE_1 %bb.1.for.body.preheader, implicit %eflags
```

This patch addresses this issue and make newly created branch instructions to keep debug-location info.

Reviewers: aprantl, MatzeB, craig.topper, qcolombet

Reviewed By: qcolombet

Subscribers: qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D29596

llvm-svn: 294976

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -23,6 +23,7 @@
 #include "llvm/CodeGen/SlotIndexes.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/ModuleSlotTracker.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
@@ -423,7 +424,7 @@ void MachineBasicBlock::updateTerminator() {
 
   MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
   SmallVector<MachineOperand, 4> Cond;
-  DebugLoc DL;  // FIXME: this is nowhere
+  DebugLoc DL = findBranchDebugLoc();
   bool B = TII->analyzeBranch(*this, TBB, FBB, Cond);
   (void) B;
   assert(!B && "UpdateTerminators requires analyzable predecessors!");
@@ -491,7 +492,7 @@ void MachineBasicBlock::updateTerminator() {
       // FIXME: This does not seem like a reasonable pattern to support, but it
       // has been seen in the wild coming out of degenerate ARM test cases.
       TII->removeBranch(*this);
-  
+
       // Finally update the unconditional successor to be reached via a branch if
       // it would not be reached by fallthrough.
       if (!isLayoutSuccessor(TBB))
@@ -1150,6 +1151,24 @@ MachineBasicBlock::findDebugLoc(instr_iterator MBBI) {
   return {};
 }
 
+/// Find and return the merged DebugLoc of the branch instructions of the block.
+/// Return UnknownLoc if there is none.
+DebugLoc
+MachineBasicBlock::findBranchDebugLoc() {
+  DebugLoc DL {};
+  auto TI = getFirstTerminator();
+  while (TI != end() && !TI->isBranch())
+    ++TI;
+
+  if (TI != end()) {
+    DL = TI->getDebugLoc();
+    for (++TI ; TI != end() ; ++TI)
+      if (TI->isBranch())
+        DL = DILocation::getMergedLocation(DL, TI->getDebugLoc());
+  }
+  return DL;
+}
+
 /// Return probability of the edge from this block to MBB.
 BranchProbability
 MachineBasicBlock::getSuccProbability(const_succ_iterator Succ) const {