r330566 - [Atomics] warn about atomic accesses using libcalls

[Tim Northover via cfe-commits]

Author: tnorthover
Date: Mon Apr 23 01:16:24 2018
New Revision: 330566

URL: http://llvm.org/viewvc/llvm-project?rev=330566&view=rev
Log:
[Atomics] warn about atomic accesses using libcalls

If an atomic variable is misaligned (and that suspicion is why Clang emits
libcalls at all) the runtime support library will have to use a lock to safely
access it, with potentially very bad performance consequences. There's a very
good chance this is unintentional so it makes sense to issue a warning.

Also give it a named group so people can promote it to an error, or disable it
if they really don't care.

Added:
    cfe/trunk/test/CodeGen/atomics-sema-alignment.c
Modified:
    cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
    cfe/trunk/lib/CodeGen/CGAtomic.cpp

Modified: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?rev=330566&r1=330565&r2=330566&view=diff
==============================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td (original)
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td Mon Apr 23 01:16:24 2018
@@ -7111,6 +7111,9 @@ def warn_atomic_op_has_invalid_memory_or
   InGroup<DiagGroup<"atomic-memory-ordering">>;
 def err_atomic_op_has_invalid_synch_scope : Error<
   "synchronization scope argument to atomic operation is invalid">;
+def warn_atomic_op_misaligned : Warning<
+  "misaligned or large atomic operation may incur significant performance penalty">,
+  InGroup<DiagGroup<"atomic-alignment">>;
 
 def err_overflow_builtin_must_be_int : Error<
   "operand argument to overflow builtin must be an integer (%0 invalid)">;

Modified: cfe/trunk/lib/CodeGen/CGAtomic.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGAtomic.cpp?rev=330566&r1=330565&r2=330566&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGAtomic.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGAtomic.cpp Mon Apr 23 01:16:24 2018
@@ -18,6 +18,7 @@
 #include "TargetInfo.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/CodeGen/CGFunctionInfo.h"
+#include "clang/Sema/SemaDiagnostic.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Intrinsics.h"
@@ -751,6 +752,13 @@ RValue CodeGenFunction::EmitAtomicExpr(A
   Address Dest = Address::invalid();
   Address Ptr = EmitPointerWithAlignment(E->getPtr());
 
+  if (E->getOp() == AtomicExpr::AO__c11_atomic_init ||
+      E->getOp() == AtomicExpr::AO__opencl_atomic_init) {
+    LValue lvalue = MakeAddrLValue(Ptr, AtomicTy);
+    EmitAtomicInit(E->getVal1(), lvalue);
+    return RValue::get(nullptr);
+  }
+
   CharUnits sizeChars, alignChars;
   std::tie(sizeChars, alignChars) = getContext().getTypeInfoInChars(AtomicTy);
   uint64_t Size = sizeChars.getQuantity();
@@ -758,12 +766,8 @@ RValue CodeGenFunction::EmitAtomicExpr(A
   bool UseLibcall = ((Ptr.getAlignment() % sizeChars) != 0 ||
                      getContext().toBits(sizeChars) > MaxInlineWidthInBits);
 
-  if (E->getOp() == AtomicExpr::AO__c11_atomic_init ||
-      E->getOp() == AtomicExpr::AO__opencl_atomic_init) {
-    LValue lvalue = MakeAddrLValue(Ptr, AtomicTy);
-    EmitAtomicInit(E->getVal1(), lvalue);
-    return RValue::get(nullptr);
-  }
+  if (UseLibcall)
+    CGM.getDiags().Report(E->getLocStart(), diag::warn_atomic_op_misaligned);
 
   llvm::Value *Order = EmitScalarExpr(E->getOrder());
   llvm::Value *Scope =

Added: cfe/trunk/test/CodeGen/atomics-sema-alignment.c
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGen/atomics-sema-alignment.c?rev=330566&view=auto
==============================================================================
--- cfe/trunk/test/CodeGen/atomics-sema-alignment.c (added)
+++ cfe/trunk/test/CodeGen/atomics-sema-alignment.c Mon Apr 23 01:16:24 2018
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -triple aarch64-linux-gnu %s -emit-llvm -o /dev/null -verify
+
+typedef struct {
+  int a, b;
+} IntPair;
+
+typedef struct {
+  long long a;
+} LongStruct;
+
+typedef int __attribute__((aligned(1))) unaligned_int;
+
+void func(IntPair *p) {
+  IntPair res;
+  __atomic_load(p, &res, 0); // expected-warning {{misaligned or large atomic operation may incur significant performance penalty}}
+  __atomic_store(p, &res, 0); // expected-warning {{misaligned or large atomic operation may incur significant performance penalty}}
+  __atomic_fetch_add((unaligned_int *)p, 1, 2); // expected-warning {{misaligned or large atomic operation may incur significant performance penalty}}
+  __atomic_fetch_sub((unaligned_int *)p, 1, 3); // expected-warning {{misaligned or large atomic operation may incur significant performance penalty}}
+}
+
+void func1(LongStruct *p) {
+  LongStruct res;
+  __atomic_load(p, &res, 0);
+  __atomic_store(p, &res, 0);
+  __atomic_fetch_add((int *)p, 1, 2);
+  __atomic_fetch_sub((int *)p, 1, 3);
+}