Generate better location ranges for some register-described variables.

Don't terminate location ranges for register-described variables
at the end of machine basic block if this register is never modified
in the function body, except for the prologue and epilogue. Prologue
location is guessed by FrameSetup flags on MachineInstructions, while
epilogue location is deduced from debug locations of instructions
in the basic blocks ending with return instructions.

This patch is mostly targeted to fix non-trivial debug locations for
variables addressed via stack and frame pointers.

It is not really a generic fix. We can still produce poor debug info
for register-described variables if this register *is* modified somewhere
in the function, but in unrelated places. This might be the case for the debug
info in optimized binaries (e.g. for local variables in inlined functions).
LiveDebugVariables pass in CodeGen attempts to fix this problem by adjusting
DBG_VALUE instructions, but this pass is tied to greedy register allocator,
which is used in optimized builds only. Proper fix would likely involve
generalizing LiveDebugVariables to all register allocators. See more discussion
in http://reviews.llvm.org/D3933 review thread.

I'm proceeding with this patch to fix immediate severe problems and
important cases, e.g. fix completely broken debug info with AddressSanitizer
and fix PR19307 (missing debug info for by-value std::string arguments).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210492 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 55 insertions, 24 deletions

diff --git a/lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.cpp b/lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.cpp
index 61032548ea..a66d08e501 100644
--- a/lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.cpp
+++ b/lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.cpp
@@ -15,6 +15,7 @@
 #include "llvm/Target/TargetRegisterInfo.h"
 #include <algorithm>
 #include <map>
+#include <set>
 
 #define DEBUG_TYPE "dwarfdebug"
 
@@ -110,45 +111,73 @@ static void clobberRegisterUses(RegDescribedVarsMap &RegVars, unsigned RegNo,
   RegVars.erase(I);
 }
 
-// \brief Terminate location ranges for all variables, described by registers
-// clobbered by @MI.
-static void clobberRegisterUses(RegDescribedVarsMap &RegVars,
-                                const MachineInstr &MI,
-                                const TargetRegisterInfo *TRI,
-                                DbgValueHistoryMap &HistMap) {
+// \brief Collect all registers clobbered by @MI and insert them to @Regs.
+static void collectClobberedRegisters(const MachineInstr &MI,
+                                      const TargetRegisterInfo *TRI,
+                                      std::set<unsigned> &Regs) {
   for (const MachineOperand &MO : MI.operands()) {
     if (!MO.isReg() || !MO.isDef() || !MO.getReg())
       continue;
-    for (MCRegAliasIterator AI(MO.getReg(), TRI, true); AI.isValid();
-         ++AI) {
-      unsigned RegNo = *AI;
-      clobberRegisterUses(RegVars, RegNo, HistMap, MI);
-    }
+    for (MCRegAliasIterator AI(MO.getReg(), TRI, true); AI.isValid(); ++AI)
+      Regs.insert(*AI);
   }
 }
 
-// \brief Terminate the location range for all register-described variables
-// by inserting @ClobberingInstr to their history.
-static void clobberAllRegistersUses(RegDescribedVarsMap &RegVars,
-                                    DbgValueHistoryMap &HistMap,
-                                    const MachineInstr &ClobberingInstr) {
-  for (const auto &I : RegVars)
-    for (const auto &Var : I.second)
-      HistMap.endInstrRange(Var, ClobberingInstr);
-  RegVars.clear();
+// \brief Returns the first instruction in @MBB which corresponds to
+// the function epilogue, or nullptr if @MBB doesn't contain an epilogue.
+static const MachineInstr *getFirstEpilogueInst(const MachineBasicBlock &MBB) {
+  auto LastMI = MBB.getLastNonDebugInstr();
+  if (LastMI == MBB.end() || !LastMI->isReturn())
+    return nullptr;
+  // Assume that epilogue starts with instruction having the same debug location
+  // as the return instruction.
+  DebugLoc LastLoc = LastMI->getDebugLoc();
+  auto Res = LastMI;
+  for (MachineBasicBlock::const_reverse_iterator I(std::next(LastMI)); I != MBB.rend();
+       ++I) {
+    if (I->getDebugLoc() != LastLoc)
+      return Res;
+    Res = std::prev(I.base());
+  }
+  // If all instructions have the same debug location, assume whole MBB is
+  // an epilogue.
+  return MBB.begin();
+}
+
+// \brief Collect registers that are modified in the function body (their
+// contents is changed only in the prologue and epilogue).
+static void collectChangingRegs(const MachineFunction *MF,
+                                const TargetRegisterInfo *TRI,
+                                std::set<unsigned> &Regs) {
+  for (const auto &MBB : *MF) {
+    auto FirstEpilogueInst = getFirstEpilogueInst(MBB);
+    bool IsInEpilogue = false;
+    for (const auto &MI : MBB) {
+      IsInEpilogue |= &MI == FirstEpilogueInst;
+      if (!MI.getFlag(MachineInstr::FrameSetup) && !IsInEpilogue)
+        collectClobberedRegisters(MI, TRI, Regs);
+    }
+  }
 }
 
 void calculateDbgValueHistory(const MachineFunction *MF,
                               const TargetRegisterInfo *TRI,
                               DbgValueHistoryMap &Result) {
-  RegDescribedVarsMap RegVars;
+  std::set<unsigned> ChangingRegs;
+  collectChangingRegs(MF, TRI, ChangingRegs);
 
+  RegDescribedVarsMap RegVars;
   for (const auto &MBB : *MF) {
     for (const auto &MI : MBB) {
       if (!MI.isDebugValue()) {
         // Not a DBG_VALUE instruction. It may clobber registers which describe
         // some variables.
-        clobberRegisterUses(RegVars, MI, TRI, Result);
+        std::set<unsigned> MIClobberedRegs;
+        collectClobberedRegisters(MI, TRI, MIClobberedRegs);
+        for (unsigned RegNo : MIClobberedRegs) {
+          if (ChangingRegs.count(RegNo))
+            clobberRegisterUses(RegVars, RegNo, Result, MI);
+        }
         continue;
       }
 
@@ -167,8 +196,10 @@ void calculateDbgValueHistory(const MachineFunction *MF,
     // Make sure locations for register-described variables are valid only
     // until the end of the basic block (unless it's the last basic block, in
     // which case let their liveness run off to the end of the function).
-    if (!MBB.empty() &&  &MBB != &MF->back())
-      clobberAllRegistersUses(RegVars, Result, MBB.back());
+    if (!MBB.empty() &&  &MBB != &MF->back()) {
+      for (unsigned RegNo : ChangingRegs)
+        clobberRegisterUses(RegVars, RegNo, Result, MBB.back());
+    }
   }
 }