[SSP, 2/2] Create llvm.stackguard() intrinsic and lower it to LOAD_STACK_GUARD

With this change, ideally IR pass can always generate llvm.stackguard
call to get the stack guard; but for now there are still IR form stack
guard customizations around (see getIRStackGuard()). Future SSP
customization should go through LOAD_STACK_GUARD.

There is a behavior change: stack guard values are not CSEed anymore,
since we should never reuse the value in case that it has been spilled (and
corrupted). See ssp-guard-spill.ll. This also cause the change of stack
size and codegen in X86 and AArch64 test cases.

Ideally we'd like to know if the guard created in llvm.stackprotector() gets
spilled or not. If the value is spilled, discard the value and reload
stack guard; otherwise reuse the value. This can be done by teaching
register allocator to know how to rematerialize LOAD_STACK_GUARD and
force a rematerialization (which seems hard), or check for spilling in
expandPostRAPseudo. It only makes sense when the stack guard is a global
variable, which requires more instructions to load. Anyway, this seems to go out
of the scope of the current patch.

llvm-svn: 266806

1 files changed, 37 insertions, 24 deletions

diff --git a/llvm/lib/CodeGen/StackProtector.cpp b/llvm/lib/CodeGen/StackProtector.cpp
index 5407c740ead..8a56c6a14d9 100644
--- a/llvm/lib/CodeGen/StackProtector.cpp
+++ b/llvm/lib/CodeGen/StackProtector.cpp
@@ -271,36 +271,51 @@ bool StackProtector::RequiresStackProtector() {
   return NeedsProtector;
 }
 
-/// Insert code into the entry block that stores the __stack_chk_guard
+/// Create a stack guard loading and populate whether SelectionDAG SSP is
+/// supported.
+static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M,
+                            IRBuilder<> &B,
+                            bool *SupportsSelectionDAGSP = nullptr) {
+  if (Value *Guard = TLI->getIRStackGuard(B))
+    return B.CreateLoad(Guard, true, "StackGuard");
+
+  // Use SelectionDAG SSP handling, since there isn't an IR guard.
+  //
+  // This is more or less weird, since we optionally output whether we
+  // should perform a SelectionDAG SP here. The reason is that it's strictly
+  // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also
+  // mutating. There is no way to get this bit without mutating the IR, so
+  // getting this bit has to happen in this right time.
+  //
+  // We could have define a new function TLI::supportsSelectionDAGSP(), but that
+  // will put more burden on the backends' overriding work, especially when it
+  // actually conveys the same information getIRStackGuard() already gives.
+  if (SupportsSelectionDAGSP)
+    *SupportsSelectionDAGSP = true;
+  TLI->insertSSPDeclarations(*M);
+  return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard));
+}
+
+/// Insert code into the entry block that stores the stack guard
 /// variable onto the stack:
 ///
 ///   entry:
 ///     StackGuardSlot = alloca i8*
-///     StackGuard = load __stack_chk_guard
-///     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
+///     StackGuard = <stack guard>
+///     call void @llvm.stackprotector(StackGuard, StackGuardSlot)
 ///
 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
 /// node.
 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
-                           const TargetLoweringBase *TLI, AllocaInst *&AI,
-                           Value *&StackGuardVar) {
+                           const TargetLoweringBase *TLI, AllocaInst *&AI) {
   bool SupportsSelectionDAGSP = false;
   IRBuilder<> B(&F->getEntryBlock().front());
-
-  StackGuardVar = TLI->getIRStackGuard(B);
-  if (!StackGuardVar) {
-    /// Use SelectionDAG SSP handling, since there isn't an IR guard.
-    SupportsSelectionDAGSP = true;
-    TLI->insertSSPDeclarations(*M);
-    StackGuardVar = TLI->getSDStackGuard(*M);
-  }
-  assert(StackGuardVar && "Must have stack guard available");
-
   PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
   AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
-  LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
+
+  Value *Guard = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP);
   B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
-               {LI, AI});
+               {Guard, AI});
   return SupportsSelectionDAGSP;
 }
 
@@ -314,7 +329,6 @@ bool StackProtector::InsertStackProtectors() {
   bool SupportsSelectionDAGSP =
       EnableSelectionDAGSP && !TM->Options.EnableFastISel;
   AllocaInst *AI = nullptr;       // Place on stack that stores the stack guard.
-  Value *StackGuardVar = nullptr; // The stack guard variable.
 
   for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
     BasicBlock *BB = &*I++;
@@ -324,8 +338,7 @@ bool StackProtector::InsertStackProtectors() {
 
     if (!HasPrologue) {
       HasPrologue = true;
-      SupportsSelectionDAGSP &=
-          CreatePrologue(F, M, RI, TLI, AI, StackGuardVar);
+      SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI);
     }
 
     if (!SupportsSelectionDAGSP) {
@@ -342,7 +355,7 @@ bool StackProtector::InsertStackProtectors() {
       //
       //   return:
       //     ...
-      //     %1 = load __stack_chk_guard
+      //     %1 = <stack guard>
       //     %2 = load StackGuardSlot
       //     %3 = cmp i1 %1, %2
       //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
@@ -381,9 +394,9 @@ bool StackProtector::InsertStackProtectors() {
 
       // Generate the stack protector instructions in the old basic block.
       IRBuilder<> B(BB);
-      LoadInst *LI1 = B.CreateLoad(StackGuardVar);
-      LoadInst *LI2 = B.CreateLoad(AI);
-      Value *Cmp = B.CreateICmpEQ(LI1, LI2);
+      Value *Guard = getStackGuard(TLI, M, B);
+      LoadInst *LI2 = B.CreateLoad(AI, true);
+      Value *Cmp = B.CreateICmpEQ(Guard, LI2);
       auto SuccessProb =
           BranchProbabilityInfo::getBranchProbStackProtector(true);
       auto FailureProb =