Correct dwarf unwind information in function epilogue

This patch aims to provide correct dwarf unwind information in function
epilogue for X86.
It consists of two parts. The first part inserts CFI instructions that set
appropriate cfa offset and cfa register in emitEpilogue() in
X86FrameLowering. This part is X86 specific.

The second part is platform independent and ensures that:

* CFI instructions do not affect code generation (they are not counted as
  instructions when tail duplicating or tail merging)
* Unwind information remains correct when a function is modified by
  different passes. This is done in a late pass by analyzing information
  about cfa offset and cfa register in BBs and inserting additional CFI
  directives where necessary.

Added CFIInstrInserter pass:

* analyzes each basic block to determine cfa offset and register are valid
  at its entry and exit
* verifies that outgoing cfa offset and register of predecessor blocks match
  incoming values of their successors
* inserts additional CFI directives at basic block beginning to correct the
  rule for calculating CFA

Having CFI instructions in function epilogue can cause incorrect CFA
calculation rule for some basic blocks. This can happen if, due to basic
block reordering, or the existence of multiple epilogue blocks, some of the
blocks have wrong cfa offset and register values set by the epilogue block
above them.
CFIInstrInserter is currently run only on X86, but can be used by any target
that implements support for adding CFI instructions in epilogue.

Patch by Violeta Vukobrat.

Differential Revision: https://reviews.llvm.org/D42848

llvm-svn: 330706

1 files changed, 49 insertions, 12 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 7f358a67936..f160efd4a0d 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -296,6 +296,11 @@ static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
   return HashMachineInstr(*I);
 }
 
+///  Whether MI should be counted as an instruction when calculating common tail.
+static bool countsAsInstruction(const MachineInstr &MI) {
+  return !(MI.isDebugValue() || MI.isCFIInstruction());
+}
+
 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
 /// of instructions they actually have in common together at their end.  Return
 /// iterators for the first shared instruction in each block.
@@ -310,26 +315,27 @@ static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
   while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
     --I1; --I2;
     // Skip debugging pseudos; necessary to avoid changing the code.
-    while (I1->isDebugValue()) {
+    while (!countsAsInstruction(*I1)) {
       if (I1==MBB1->begin()) {
-        while (I2->isDebugValue()) {
-          if (I2==MBB2->begin())
+        while (!countsAsInstruction(*I2)) {
+          if (I2==MBB2->begin()) {
             // I1==DBG at begin; I2==DBG at begin
-            return TailLen;
+            goto SkipTopCFIAndReturn;
+          }
           --I2;
         }
         ++I2;
         // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
-        return TailLen;
+        goto SkipTopCFIAndReturn;
       }
       --I1;
     }
     // I1==first (untested) non-DBG preceding known match
-    while (I2->isDebugValue()) {
+    while (!countsAsInstruction(*I2)) {
       if (I2==MBB2->begin()) {
         ++I1;
         // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
-        return TailLen;
+        goto SkipTopCFIAndReturn;
       }
       --I2;
     }
@@ -368,6 +374,37 @@ static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
     }
     ++I1;
   }
+
+SkipTopCFIAndReturn:
+  // Ensure that I1 and I2 do not point to a CFI_INSTRUCTION. This can happen if
+  // I1 and I2 are non-identical when compared and then one or both of them ends
+  // up pointing to a CFI instruction after being incremented. For example:
+  /*
+    BB1:
+    ...
+    INSTRUCTION_A
+    ADD32ri8  <- last common instruction
+    ...
+    BB2:
+    ...
+    INSTRUCTION_B
+    CFI_INSTRUCTION
+    ADD32ri8  <- last common instruction
+    ...
+  */
+  // When INSTRUCTION_A and INSTRUCTION_B are compared as not equal, after
+  // incrementing the iterators, I1 will point to ADD, however I2 will point to
+  // the CFI instruction. Later on, this leads to BB2 being 'hacked off' at the
+  // wrong place (in ReplaceTailWithBranchTo()) which results in losing this CFI
+  // instruction.
+  while (I1 != MBB1->end() && I1->isCFIInstruction()) {
+    ++I1;
+  }
+
+  while (I2 != MBB2->end() && I2->isCFIInstruction()) {
+    ++I2;
+  }
+
   return TailLen;
 }
 
@@ -454,7 +491,7 @@ static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
                                 MachineBasicBlock::iterator E) {
   unsigned Time = 0;
   for (; I != E; ++I) {
-    if (I->isDebugValue())
+    if (!countsAsInstruction(*I))
       continue;
     if (I->isCall())
       Time += 10;
@@ -814,12 +851,12 @@ mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
     assert(MBBI != MBBIE && "Reached BB end within common tail length!");
     (void)MBBIE;
 
-    if (MBBI->isDebugValue()) {
+    if (!countsAsInstruction(*MBBI)) {
       ++MBBI;
       continue;
     }
 
-    while ((MBBICommon != MBBIECommon) && MBBICommon->isDebugValue())
+    while ((MBBICommon != MBBIECommon) && !countsAsInstruction(*MBBICommon))
       ++MBBICommon;
 
     assert(MBBICommon != MBBIECommon &&
@@ -859,7 +896,7 @@ void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
   }
 
   for (auto &MI : *MBB) {
-    if (MI.isDebugValue())
+    if (!countsAsInstruction(MI))
       continue;
     DebugLoc DL = MI.getDebugLoc();
     for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) {
@@ -869,7 +906,7 @@ void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
       auto &Pos = NextCommonInsts[i];
       assert(Pos != SameTails[i].getBlock()->end() &&
           "Reached BB end within common tail");
-      while (Pos->isDebugValue()) {
+      while (!countsAsInstruction(*Pos)) {
         ++Pos;
         assert(Pos != SameTails[i].getBlock()->end() &&
             "Reached BB end within common tail");