[BOLT] Identify indirect tail call

[liusy58]

(1)Use CFI directives OpDefCfaOffset 0 to identify indirect tail call like br x*. OpDefCfaOffset 0 implies that the stack frame from the caller to the target function remains unchanged. So if a br x* instruction is preceded by a OpDefCfaOffset 0 directive, we can conclude that it is an indirect tail call, under the assumption that no other instructions have modified the stack pointer (sp) between the OpDefCfaOffset 0 directive and the br x* instruction. (2)If there are no instructions manipulating the stack within a function, we can conclude that br x* is an indirect tail call.

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Nicholas (liusy58)

Changes

(1)Use CFI directives OpDefCfaOffset 0 to identify indirect tail call like br x*. OpDefCfaOffset 0 implies that the stack frame from the caller to the target function remains unchanged. So if a br x* instruction is preceded by a OpDefCfaOffset 0 directive, we can conclude that it is an indirect tail call, under the assumption that no other instructions have modified the stack pointer (sp) between the OpDefCfaOffset 0 directive and the br x* instruction. (2)If there are no instructions manipulating the stack within a function, we can conclude that br x* is an indirect tail call.

---

Full diff: https://github.com/llvm/llvm-project/pull/121146.diff

4 Files Affected:

• (modified) bolt/include/bolt/Core/MCPlusBuilder.h (+5)
• (modified) bolt/lib/Core/BinaryFunction.cpp (+33)
• (modified) bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp (+10)
• (modified) bolt/lib/Target/X86/X86MCPlusBuilder.cpp (+11)

diff --git a/bolt/include/bolt/Core/MCPlusBuilder.h b/bolt/include/bolt/Core/MCPlusBuilder.h
index 3634fed9757ceb..b4a284fba19a6e 100644
--- a/bolt/include/bolt/Core/MCPlusBuilder.h
+++ b/bolt/include/bolt/Core/MCPlusBuilder.h
@@ -610,6 +610,11 @@ class MCPlusBuilder {
 
   virtual bool isLeave(const MCInst &Inst) const { return false; }
 
+  virtual bool hasUseOrDefofSPOrFP(const MCInst &Inst) const {
+    llvm_unreachable("not implemented");
+    return false;
+  }
+
   virtual bool isADRP(const MCInst &Inst) const {
     llvm_unreachable("not implemented");
     return false;
diff --git a/bolt/lib/Core/BinaryFunction.cpp b/bolt/lib/Core/BinaryFunction.cpp
index 1c5cd62a095b24..e137effce37dcb 100644
--- a/bolt/lib/Core/BinaryFunction.cpp
+++ b/bolt/lib/Core/BinaryFunction.cpp
@@ -1961,6 +1961,39 @@ bool BinaryFunction::postProcessIndirectBranches(
       bool IsEpilogue = llvm::any_of(BB, [&](const MCInst &Instr) {
         return BC.MIB->isLeave(Instr) || BC.MIB->isPop(Instr);
       });
+      // Any adr instruction of aarch64 will generate a new entry,
+      // Adr instruction cannt afford to do any optimizations
+      if (!IsEpilogue && !isMultiEntry()) {
+        BinaryBasicBlock::iterator LastDefCFAOffsetInstIter = BB.end();
+        // find the last OpDefCfaOffset 0 instruction.
+        for (BinaryBasicBlock::iterator Iter = BB.begin(); Iter != BB.end();
+             ++Iter) {
+          if (&*Iter == &Instr) {
+            break;
+          }
+          if (BC.MIB->isCFI(*Iter)) {
+            const MCCFIInstruction *CFIInst = BB.getParent()->getCFIFor(*Iter);
+            if ((CFIInst->getOperation() == MCCFIInstruction::OpDefCfaOffset) &&
+                (CFIInst->getOffset() == 0)) {
+              LastDefCFAOffsetInstIter = Iter;
+              break;
+            }
+          }
+        }
+        if (LastDefCFAOffsetInstIter != BB.end()) {
+          IsEpilogue = true;
+          // make sure there is no instruction manipulating sp between the two
+          // instructions
+          BinaryBasicBlock::iterator Iter = LastDefCFAOffsetInstIter;
+          while (&*Iter != &Instr) {
+            if (BC.MIB->hasUseOrDefofSPOrFP(*Iter)) {
+              IsEpilogue = false;
+              break;
+            }
+            ++Iter;
+          }
+        }
+      }
       if (IsEpilogue) {
         BC.MIB->convertJmpToTailCall(Instr);
         BB.removeAllSuccessors();
diff --git a/bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp b/bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp
index 7e08e5c81d26ff..91a8f755b9aa47 100644
--- a/bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp
+++ b/bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp
@@ -1790,6 +1790,16 @@ class AArch64MCPlusBuilder : public MCPlusBuilder {
   }
 
   uint16_t getMinFunctionAlignment() const override { return 4; }
+
+  bool hasUseOrDefofSPOrFP(const MCInst &Inst) const override {
+    if (isPseudo(Inst) || isNoop(Inst) || isCFI(Inst)) {
+      return false;
+    }
+    return hasDefOfPhysReg(Inst, AArch64::SP) ||
+           hasUseOfPhysReg(Inst, AArch64::SP) ||
+           hasDefOfPhysReg(Inst, AArch64::FP) ||
+           hasUseOfPhysReg(Inst, AArch64::FP);
+  }
 };
 
 } // end anonymous namespace
diff --git a/bolt/lib/Target/X86/X86MCPlusBuilder.cpp b/bolt/lib/Target/X86/X86MCPlusBuilder.cpp
index 63086c06d74fd9..ad998d8601e26f 100644
--- a/bolt/lib/Target/X86/X86MCPlusBuilder.cpp
+++ b/bolt/lib/Target/X86/X86MCPlusBuilder.cpp
@@ -89,6 +89,17 @@ class X86MCPlusBuilder : public MCPlusBuilder {
 public:
   using MCPlusBuilder::MCPlusBuilder;
 
+  virtual bool hasUseOrDefofSPOrFP(const MCInst &Inst) const override {
+    bool IsLoad, IsStore, IsStoreFromReg, IsSimple, IsIndexed;
+    MCPhysReg Reg;
+    int32_t SrcImm;
+    uint16_t StackPtrReg;
+    int64_t StackOffset;
+    uint8_t Size;
+    return isStackAccess(Inst, IsLoad, IsStore, IsStoreFromReg, Reg, SrcImm,
+                         StackPtrReg, StackOffset, Size, IsSimple, IsIndexed);
+  }
+
   std::unique_ptr<MCSymbolizer>
   createTargetSymbolizer(BinaryFunction &Function,
                          bool CreateNewSymbols) const override {

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