[PATCH] D135500: [Clang] reject bit-fields as instruction operands in Microsoft style inline asm blocks.

[Tom Honermann via Phabricator via cfe-commits]

tahonermann created this revision.
tahonermann added reviewers: erichkeane, aaron.ballman.
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MSVC allows bit-fields to be specified as instruction operands in inline asm blocks. Such references are resolved to the address of the allocation unit that the named bitfield is a part of. The result is that reads and writes of such operands will read or mutate nearby bit-fields stored in the same allocation unit. This is a surprising behavior for which MSVC issues warning C4401, "'<identifier>': member is bit field". Intel's icc compiler also allows such bit-field references, but does not issue a diagnostic.

Prior to this change, Clang fails the following assertion when a bit-field is referenced in such instructions:

  clang/lib/CodeGen/CGValue.h:338: llvm::Value* clang::CodeGen::LValue::getPointer(clang::CodeGen::CodeGenFunction&) const: Assertion `isSimple()' failed.

In non-assert enabled builds, Clang's behavior appears to match the behavior of the MSVC and icc compilers, though it is unknown if that is by design or happenstance.

Following this change, attempts to use a bit-field as an instruction operand in Microsoft style asm blocks is diagnosed as an error due to the risk of unintentionally reading or writing nearby bit-fields.

Fixes https://github.com/llvm/llvm-project/issues/57791


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D135500

Files:
  clang/include/clang/Basic/DiagnosticCommonKinds.td
  clang/lib/Sema/SemaStmtAsm.cpp
  clang/test/Parser/ms-inline-asm.c


Index: clang/test/Parser/ms-inline-asm.c
===================================================================
--- clang/test/Parser/ms-inline-asm.c
+++ clang/test/Parser/ms-inline-asm.c
@@ -62,6 +62,18 @@
   __asm mov eax, offset A // expected-error {{offset operator cannot yet handle constants}}
 }
 
+// GH57791
+typedef struct S {
+  unsigned bf1:1; // expected-note {{bit-field is declared here}}
+  unsigned bf2:1; // expected-note {{bit-field is declared here}}
+} S;
+void t15(S s) {
+  __asm {
+    mov eax, s.bf1 // expected-error {{bit-fields are not supported as operands in inline asm blocks}}
+    mov s.bf2, eax // expected-error {{bit-fields are not supported as operands in inline asm blocks}}
+  }
+}
+
 int t_fail(void) { // expected-note {{to match this}}
   __asm 
   __asm { // expected-error 3 {{expected}} expected-note {{to match this}}
Index: clang/lib/Sema/SemaStmtAsm.cpp
===================================================================
--- clang/lib/Sema/SemaStmtAsm.cpp
+++ clang/lib/Sema/SemaStmtAsm.cpp
@@ -932,13 +932,23 @@
   bool IsSimple = (NumOutputs != 0 || NumInputs != 0);
   setFunctionHasBranchProtectedScope();
 
+  bool InvalidOperand = false;
   for (uint64_t I = 0; I < NumOutputs + NumInputs; ++I) {
-    if (Exprs[I]->getType()->isBitIntType())
-      return StmtError(
-          Diag(Exprs[I]->getBeginLoc(), diag::err_asm_invalid_type)
+    if (Exprs[I]->getType()->isBitIntType()) {
+      InvalidOperand = true;
+      Diag(Exprs[I]->getBeginLoc(), diag::err_asm_invalid_type)
           << Exprs[I]->getType() << (I < NumOutputs)
-          << Exprs[I]->getSourceRange());
+          << Exprs[I]->getSourceRange();
+    } else if (Exprs[I]->refersToBitField()) {
+      InvalidOperand = true;
+      FieldDecl *BitField = Exprs[I]->getSourceBitField();
+      Diag(Exprs[I]->getBeginLoc(), diag::err_ms_asm_bitfield_unsupported)
+          << Exprs[I]->getSourceRange();
+      Diag(BitField->getLocation(), diag::note_bitfield_decl);
+    }
   }
+  if (InvalidOperand)
+    return StmtError();
 
   MSAsmStmt *NS =
     new (Context) MSAsmStmt(Context, AsmLoc, LBraceLoc, IsSimple,
Index: clang/include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticCommonKinds.td
+++ clang/include/clang/Basic/DiagnosticCommonKinds.td
@@ -279,6 +279,9 @@
   def err_asm_invalid_type : Error<
     "invalid type %0 in asm %select{input|output}1">;
 
+  def err_ms_asm_bitfield_unsupported : Error<
+    "bit-fields are not supported as operands in inline asm blocks">;
+
   def warn_stack_clash_protection_inline_asm : Warning<
     "Unable to protect inline asm that clobbers stack pointer against stack clash">,
     InGroup<DiagGroup<"stack-protector">>;


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