MS inline asm: Allow __asm blocks to set a return value

If control falls off the end of a function after an __asm block, MSVC
assumes that the inline assembly filled the EAX and possibly EDX
registers with an appropriate return value. This functionality is used
in inline functions returning 64-bit integers in system headers, so we
need some amount of compatibility.

This is implemented in Clang by adding extra output constraints to every
inline asm block, and storing the resulting output registers into the
return value slot. If we see an asm block somewhere in the function
body, we emit a normal epilogue instead of marking the end of the
function with a return type unreachable.

Normal returns in functions not using this functionality will overwrite
the return value slot, and in most cases LLVM should be able to
eliminate the dead stores.

Fixes PR17201.

Reviewed By: majnemer

Differential Revision: http://reviews.llvm.org/D5177

llvm-svn: 217187

1 files changed, 88 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 0da0e0843b9..9bf1dbbd0d8 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -15,6 +15,7 @@
 #include "TargetInfo.h"
 #include "ABIInfo.h"
 #include "CGCXXABI.h"
+#include "CGValue.h"
 #include "CodeGenFunction.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/CodeGen/CGFunctionInfo.h"
@@ -593,6 +594,14 @@ public:
     return X86AdjustInlineAsmType(CGF, Constraint, Ty);
   }
 
+  void addReturnRegisterOutputs(CodeGenFunction &CGF, LValue ReturnValue,
+                                std::string &Constraints,
+                                std::vector<llvm::Type *> &ResultRegTypes,
+                                std::vector<llvm::Type *> &ResultTruncRegTypes,
+                                std::vector<LValue> &ResultRegDests,
+                                std::string &AsmString,
+                                unsigned NumOutputs) const override;
+
   llvm::Constant *
   getUBSanFunctionSignature(CodeGen::CodeGenModule &CGM) const override {
     unsigned Sig = (0xeb << 0) |  // jmp rel8
@@ -606,6 +615,85 @@ public:
 
 }
 
+/// Rewrite input constraint references after adding some output constraints.
+/// In the case where there is one output and one input and we add one output,
+/// we need to replace all operand references greater than or equal to 1:
+///     mov $0, $1
+///     mov eax, $1
+/// The result will be:
+///     mov $0, $2
+///     mov eax, $2
+static void rewriteInputConstraintReferences(unsigned FirstIn,
+                                             unsigned NumNewOuts,
+                                             std::string &AsmString) {
+  std::string Buf;
+  llvm::raw_string_ostream OS(Buf);
+  size_t Pos = 0;
+  while (Pos < AsmString.size()) {
+    size_t DollarStart = AsmString.find('$', Pos);
+    if (DollarStart == std::string::npos)
+      DollarStart = AsmString.size();
+    size_t DollarEnd = AsmString.find_first_not_of('$', DollarStart);
+    if (DollarEnd == std::string::npos)
+      DollarEnd = AsmString.size();
+    OS << StringRef(&AsmString[Pos], DollarEnd - Pos);
+    Pos = DollarEnd;
+    size_t NumDollars = DollarEnd - DollarStart;
+    if (NumDollars % 2 != 0 && Pos < AsmString.size()) {
+      // We have an operand reference.
+      size_t DigitStart = Pos;
+      size_t DigitEnd = AsmString.find_first_not_of("0123456789", DigitStart);
+      if (DigitEnd == std::string::npos)
+        DigitEnd = AsmString.size();
+      StringRef OperandStr(&AsmString[DigitStart], DigitEnd - DigitStart);
+      unsigned OperandIndex;
+      if (!OperandStr.getAsInteger(10, OperandIndex)) {
+        if (OperandIndex >= FirstIn)
+          OperandIndex += NumNewOuts;
+        OS << OperandIndex;
+      } else {
+        OS << OperandStr;
+      }
+      Pos = DigitEnd;
+    }
+  }
+  AsmString = std::move(OS.str());
+}
+
+/// Add output constraints for EAX:EDX because they are return registers.
+void X86_32TargetCodeGenInfo::addReturnRegisterOutputs(
+    CodeGenFunction &CGF, LValue ReturnSlot, std::string &Constraints,
+    std::vector<llvm::Type *> &ResultRegTypes,
+    std::vector<llvm::Type *> &ResultTruncRegTypes,
+    std::vector<LValue> &ResultRegDests, std::string &AsmString,
+    unsigned NumOutputs) const {
+  uint64_t RetWidth = CGF.getContext().getTypeSize(ReturnSlot.getType());
+
+  // Use the EAX constraint if the width is 32 or smaller and EAX:EDX if it is
+  // larger.
+  if (!Constraints.empty())
+    Constraints += ',';
+  if (RetWidth <= 32) {
+    Constraints += "={eax}";
+    ResultRegTypes.push_back(CGF.Int32Ty);
+  } else {
+    // Use the 'A' constraint for EAX:EDX.
+    Constraints += "=A";
+    ResultRegTypes.push_back(CGF.Int64Ty);
+  }
+
+  // Truncate EAX or EAX:EDX to an integer of the appropriate size.
+  llvm::Type *CoerceTy = llvm::IntegerType::get(CGF.getLLVMContext(), RetWidth);
+  ResultTruncRegTypes.push_back(CoerceTy);
+
+  // Coerce the integer by bitcasting the return slot pointer.
+  ReturnSlot.setAddress(CGF.Builder.CreateBitCast(ReturnSlot.getAddress(),
+                                                  CoerceTy->getPointerTo()));
+  ResultRegDests.push_back(ReturnSlot);
+
+  rewriteInputConstraintReferences(NumOutputs, 1, AsmString);
+}
+
 /// shouldReturnTypeInRegister - Determine if the given type should be
 /// passed in a register (for the Darwin ABI).
 bool X86_32ABIInfo::shouldReturnTypeInRegister(QualType Ty,