[PATCH] D28213: [Frontend] Correct values of ATOMIC_*_LOCK_FREE to match builtin

Michał Górny via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Sun Jan 1 14:57:31 PST 2017


mgorny created this revision.
mgorny added reviewers: rsmith, t.p.northover, hfinkel, EricWF.
mgorny added a subscriber: cfe-commits.

Correct the logic used to set `ATOMIC_*_LOCK_FREE` preprocessor macros not
to rely on the ABI alignment of types. Instead, just assume all those
types are aligned correctly by default since clang uses safe alignment
for `_Atomic` types even if the underlying types are aligned to a lower
boundary by default.

For example, the `long long` and `double` types on x86 are aligned to
32-bit boundary by default. However, `_Atomic long long` and `_Atomic
double` are aligned to 64-bit boundary, therefore satisfying
the requirements of lock-free atomic operations.

This fixes PR #19355 by correcting the value of
`__GCC_ATOMIC_LLONG_LOCK_FREE` on x86, and therefore also fixing
the assumption made in libc++ tests. This also fixes PR #30581 by
applying a consistent logic between the functions used to implement
both interfaces.


https://reviews.llvm.org/D28213

Files:
  lib/Frontend/InitPreprocessor.cpp
  test/Sema/atomic-ops.c


Index: test/Sema/atomic-ops.c
===================================================================
--- test/Sema/atomic-ops.c
+++ test/Sema/atomic-ops.c
@@ -14,11 +14,7 @@
 _Static_assert(__GCC_ATOMIC_SHORT_LOCK_FREE == 2, "");
 _Static_assert(__GCC_ATOMIC_INT_LOCK_FREE == 2, "");
 _Static_assert(__GCC_ATOMIC_LONG_LOCK_FREE == 2, "");
-#ifdef __i386__
-_Static_assert(__GCC_ATOMIC_LLONG_LOCK_FREE == 1, "");
-#else
 _Static_assert(__GCC_ATOMIC_LLONG_LOCK_FREE == 2, "");
-#endif
 _Static_assert(__GCC_ATOMIC_POINTER_LOCK_FREE == 2, "");
 
 _Static_assert(__c11_atomic_is_lock_free(1), "");
Index: lib/Frontend/InitPreprocessor.cpp
===================================================================
--- lib/Frontend/InitPreprocessor.cpp
+++ lib/Frontend/InitPreprocessor.cpp
@@ -286,12 +286,12 @@
 
 /// Get the value the ATOMIC_*_LOCK_FREE macro should have for a type with
 /// the specified properties.
-static const char *getLockFreeValue(unsigned TypeWidth, unsigned TypeAlign,
-                                    unsigned InlineWidth) {
+static const char *getLockFreeValue(unsigned TypeWidth, unsigned InlineWidth) {
   // Fully-aligned, power-of-2 sizes no larger than the inline
   // width will be inlined as lock-free operations.
-  if (TypeWidth == TypeAlign && (TypeWidth & (TypeWidth - 1)) == 0 &&
-      TypeWidth <= InlineWidth)
+  // Note: we do not need to check alignment since _Atomic(T) is always
+  // appropriately-aligned in clang.
+  if ((TypeWidth & (TypeWidth - 1)) == 0 && TypeWidth <= InlineWidth)
     return "2"; // "always lock free"
   // We cannot be certain what operations the lib calls might be
   // able to implement as lock-free on future processors.
@@ -881,7 +881,6 @@
 #define DEFINE_LOCK_FREE_MACRO(TYPE, Type) \
     Builder.defineMacro("__GCC_ATOMIC_" #TYPE "_LOCK_FREE", \
                         getLockFreeValue(TI.get##Type##Width(), \
-                                         TI.get##Type##Align(), \
                                          InlineWidthBits));
     DEFINE_LOCK_FREE_MACRO(BOOL, Bool);
     DEFINE_LOCK_FREE_MACRO(CHAR, Char);
@@ -894,7 +893,6 @@
     DEFINE_LOCK_FREE_MACRO(LLONG, LongLong);
     Builder.defineMacro("__GCC_ATOMIC_POINTER_LOCK_FREE",
                         getLockFreeValue(TI.getPointerWidth(0),
-                                         TI.getPointerAlign(0),
                                          InlineWidthBits));
 #undef DEFINE_LOCK_FREE_MACRO
   }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D28213.82791.patch
Type: text/x-patch
Size: 2451 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20170101/966da8b4/attachment.bin>


More information about the cfe-commits mailing list