17 files changed, 279 insertions, 2 deletions

diff --git a/gcc/c-family/c-cppbuiltin.c b/gcc/c-family/c-cppbuiltin.c
index 48cbefd..ce88e70 100644
--- a/gcc/c-family/c-cppbuiltin.c
+++ b/gcc/c-family/c-cppbuiltin.c
@@ -741,6 +741,20 @@ cpp_atomic_builtins (cpp_reader *pfile)
   builtin_define_with_int_value ("__GCC_ATOMIC_TEST_AND_SET_TRUEVAL",
 				 targetm.atomic_test_and_set_trueval);
 
+  /* Macros for C++17 hardware interference size constants.  Either both or
+     neither should be set.  */
+  gcc_assert (!param_destruct_interfere_size
+	      == !param_construct_interfere_size);
+  if (param_destruct_interfere_size)
+    {
+      /* FIXME The way of communicating these values to the library should be
+	 part of the C++ ABI, whether macro or builtin.  */
+      builtin_define_with_int_value ("__GCC_DESTRUCTIVE_SIZE",
+				     param_destruct_interfere_size);
+      builtin_define_with_int_value ("__GCC_CONSTRUCTIVE_SIZE",
+				     param_construct_interfere_size);
+    }
+
   /* ptr_type_node can't be used here since ptr_mode is only set when
      toplev calls backend_init which is not done with -E  or pch.  */
   psize = POINTER_SIZE_UNITS;
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index c5fe900..9c151d1 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -722,6 +722,11 @@ Winit-list-lifetime
 C++ ObjC++ Var(warn_init_list) Warning Init(1)
 Warn about uses of std::initializer_list that can result in dangling pointers.
 
+Winterference-size
+C++ ObjC++ Var(warn_interference_size) Warning Init(1)
+Warn about nonsensical values of --param destructive-interference-size or
+constructive-interference-size.
+
 Wimplicit
 C ObjC Var(warn_implicit) Warning LangEnabledBy(C ObjC,Wall)
 Warn about implicit declarations.
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 30d9a0b..36519cc 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -16540,6 +16540,28 @@ aarch64_override_options_internal (struct gcc_options *opts)
     SET_OPTION_IF_UNSET (opts, &global_options_set,
 			 param_l1_cache_line_size,
 			 aarch64_tune_params.prefetch->l1_cache_line_size);
+
+  if (aarch64_tune_params.prefetch->l1_cache_line_size >= 0)
+    {
+      SET_OPTION_IF_UNSET (opts, &global_options_set,
+			   param_destruct_interfere_size,
+			   aarch64_tune_params.prefetch->l1_cache_line_size);
+      SET_OPTION_IF_UNSET (opts, &global_options_set,
+			   param_construct_interfere_size,
+			   aarch64_tune_params.prefetch->l1_cache_line_size);
+    }
+  else
+    {
+      /* For a generic AArch64 target, cover the current range of cache line
+	 sizes.  */
+      SET_OPTION_IF_UNSET (opts, &global_options_set,
+			   param_destruct_interfere_size,
+			   256);
+      SET_OPTION_IF_UNSET (opts, &global_options_set,
+			   param_construct_interfere_size,
+			   64);
+    }
+
   if (aarch64_tune_params.prefetch->l2_cache_size >= 0)
     SET_OPTION_IF_UNSET (opts, &global_options_set,
 			 param_l2_cache_size,
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index f1e6282..6c6e77f 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -3669,6 +3669,28 @@ arm_option_override (void)
     SET_OPTION_IF_UNSET (&global_options, &global_options_set,
 			 param_l1_cache_line_size,
 			 current_tune->prefetch.l1_cache_line_size);
+  if (current_tune->prefetch.l1_cache_line_size >= 0)
+    {
+      SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+			   param_destruct_interfere_size,
+			   current_tune->prefetch.l1_cache_line_size);
+      SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+			   param_construct_interfere_size,
+			   current_tune->prefetch.l1_cache_line_size);
+    }
+  else
+    {
+      /* For a generic ARM target, JF Bastien proposed using 64 for both.  */
+      /* ??? Cortex A9 has a 32-byte cache line, so why not 32 for
+	 constructive?  */
+      /* More recent Cortex chips have a 64-byte cache line, but are marked
+	 ARM_PREFETCH_NOT_BENEFICIAL, so they get these defaults.  */
+      SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+			   param_destruct_interfere_size, 64);
+      SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+			   param_construct_interfere_size, 64);
+    }
+
   if (current_tune->prefetch.l1_cache_size >= 0)
     SET_OPTION_IF_UNSET (&global_options, &global_options_set,
 			 param_l1_cache_size,
diff --git a/gcc/config/i386/i386-options.c b/gcc/config/i386/i386-options.c
index 2cb87ce..c0006b3 100644
--- a/gcc/config/i386/i386-options.c
+++ b/gcc/config/i386/i386-options.c
@@ -2579,6 +2579,12 @@ ix86_option_override_internal (bool main_args_p,
   SET_OPTION_IF_UNSET (opts, opts_set, param_l2_cache_size,
 		       ix86_tune_cost->l2_cache_size);
 
+  /* 64B is the accepted value for these for all x86.  */
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+		       param_destruct_interfere_size, 64);
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+		       param_construct_interfere_size, 64);
+
   /* Enable sw prefetching at -O3 for CPUS that prefetching is helpful.  */
   if (opts->x_flag_prefetch_loop_arrays < 0
       && HAVE_prefetch
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 7772fe6..0c2498a 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6075,6 +6075,37 @@ inline_asm_in_constexpr_error (location_t loc)
 	  "%<constexpr%> function in C++20");
 }
 
+/* We're getting the constant value of DECL in a manifestly constant-evaluated
+   context; maybe complain about that.  */
+
+static void
+maybe_warn_about_constant_value (location_t loc, tree decl)
+{
+  static bool explained = false;
+  if (cxx_dialect >= cxx17
+      && warn_interference_size
+      && !global_options_set.x_param_destruct_interfere_size
+      && DECL_CONTEXT (decl) == std_node
+      && id_equal (DECL_NAME (decl), "hardware_destructive_interference_size")
+      && (LOCATION_FILE (input_location) != main_input_filename
+	  || module_exporting_p ())
+      && warning_at (loc, OPT_Winterference_size, "use of %qD", decl)
+      && !explained)
+    {
+      explained = true;
+      inform (loc, "its value can vary between compiler versions or "
+	      "with different %<-mtune%> or %<-mcpu%> flags");
+      inform (loc, "if this use is part of a public ABI, change it to "
+	      "instead use a constant variable you define");
+      inform (loc, "the default value for the current CPU tuning "
+	      "is %d bytes", param_destruct_interfere_size);
+      inform (loc, "you can stabilize this value with %<--param "
+	      "hardware_destructive_interference_size=%d%>, or disable "
+	      "this warning with %<-Wno-interference-size%>",
+	      param_destruct_interfere_size);
+    }
+}
+
 /* Attempt to reduce the expression T to a constant value.
    On failure, issue diagnostic and return error_mark_node.  */
 /* FIXME unify with c_fully_fold */
@@ -6219,6 +6250,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	      r = *p;
 	      break;
 	    }
+      if (ctx->manifestly_const_eval)
+	maybe_warn_about_constant_value (loc, t);
       if (COMPLETE_TYPE_P (TREE_TYPE (t))
 	  && is_really_empty_class (TREE_TYPE (t), /*ignore_vptr*/false))
 	{
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index bce62ad..c206502 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -4752,6 +4752,38 @@ cxx_init_decl_processing (void)
   /* Show we use EH for cleanups.  */
   if (flag_exceptions)
     using_eh_for_cleanups ();
+
+  /* Check that the hardware interference sizes are at least
+     alignof(max_align_t), as required by the standard.  */
+  const int max_align = max_align_t_align () / BITS_PER_UNIT;
+  if (param_destruct_interfere_size)
+    {
+      if (param_destruct_interfere_size < max_align)
+	error ("%<--param destructive-interference-size=%d%> is less than "
+	       "%d", param_destruct_interfere_size, max_align);
+      else if (param_destruct_interfere_size < param_l1_cache_line_size)
+	warning (OPT_Winterference_size,
+		 "%<--param destructive-interference-size=%d%> "
+		 "is less than %<--param l1-cache-line-size=%d%>",
+		 param_destruct_interfere_size, param_l1_cache_line_size);
+    }
+  else if (param_l1_cache_line_size >= max_align)
+    param_destruct_interfere_size = param_l1_cache_line_size;
+  /* else leave it unset.  */
+
+  if (param_construct_interfere_size)
+    {
+      if (param_construct_interfere_size < max_align)
+	error ("%<--param constructive-interference-size=%d%> is less than "
+	       "%d", param_construct_interfere_size, max_align);
+      else if (param_construct_interfere_size > param_l1_cache_line_size)
+	warning (OPT_Winterference_size,
+		 "%<--param constructive-interference-size=%d%> "
+		 "is greater than %<--param l1-cache-line-size=%d%>",
+		 param_construct_interfere_size, param_l1_cache_line_size);
+    }
+  else if (param_l1_cache_line_size >= max_align)
+    param_construct_interfere_size = param_l1_cache_line_size;
 }
 
 /* Enter an abi node in global-module context.  returns a cookie to
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 23cc68f..78cfc10 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -9018,6 +9018,43 @@ that has already been done in the current function.  Therefore,
 seemingly insignificant changes in the source program can cause the
 warnings produced by @option{-Winline} to appear or disappear.
 
+@item -Winterference-size
+@opindex Winterference-size
+Warn about use of C++17 @code{std::hardware_destructive_interference_size}
+without specifying its value with @option{--param destructive-interference-size}.
+Also warn about questionable values for that option.
+
+This variable is intended to be used for controlling class layout, to
+avoid false sharing in concurrent code:
+
+@smallexample
+struct independent_fields @{
+  alignas(std::hardware_destructive_interference_size) std::atomic<int> one;
+  alignas(std::hardware_destructive_interference_size) std::atomic<int> two;
+@};
+@end smallexample
+
+Here @samp{one} and @samp{two} are intended to be far enough apart
+that stores to one won't require accesses to the other to reload the
+cache line.
+
+By default, @option{--param destructive-interference-size} and
+@option{--param constructive-interference-size} are set based on the
+current @option{-mtune} option, typically to the L1 cache line size
+for the particular target CPU, sometimes to a range if tuning for a
+generic target.  So all translation units that depend on ABI
+compatibility for the use of these variables must be compiled with
+the same @option{-mtune} (or @option{-mcpu}).
+
+If ABI stability is important, such as if the use is in a header for a
+library, you should probably not use the hardware interference size
+variables at all.  Alternatively, you can force a particular value
+with @option{--param}.
+
+If you are confident that your use of the variable does not affect ABI
+outside a single build of your project, you can turn off the warning
+with @option{-Wno-interference-size}.
+
 @item -Wint-in-bool-context
 @opindex Wint-in-bool-context
 @opindex Wno-int-in-bool-context
@@ -13938,6 +13975,34 @@ prefetch hints can be issued for any constant stride.
 
 This setting is only useful for strides that are known and constant.
 
+@item destructive-interference-size
+@item constructive-interference-size
+The values for the C++17 variables
+@code{std::hardware_destructive_interference_size} and
+@code{std::hardware_constructive_interference_size}.  The destructive
+interference size is the minimum recommended offset between two
+independent concurrently-accessed objects; the constructive
+interference size is the maximum recommended size of contiguous memory
+accessed together.  Typically both will be the size of an L1 cache
+line for the target, in bytes.  For a generic target covering a range of L1
+cache line sizes, typically the constructive interference size will be
+the small end of the range and the destructive size will be the large
+end.
+
+The destructive interference size is intended to be used for layout,
+and thus has ABI impact.  The default value is not expected to be
+stable, and on some targets varies with @option{-mtune}, so use of
+this variable in a context where ABI stability is important, such as
+the public interface of a library, is strongly discouraged; if it is
+used in that context, users can stabilize the value using this
+option.
+
+The constructive interference size is less sensitive, as it is
+typically only used in a @samp{static_assert} to make sure that a type
+fits within a cache line.
+
+See also @option{-Winterference-size}.
+
 @item loop-interchange-max-num-stmts
 The maximum number of stmts in a loop to be interchanged.
 
diff --git a/gcc/params.opt b/gcc/params.opt
index 3a701e2..658ca02 100644
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -361,6 +361,22 @@ The maximum code size growth ratio when expanding into a jump table (in percent)
 Common Joined UInteger Var(param_l1_cache_line_size) Init(32) Param Optimization
 The size of L1 cache line.
 
+-param=destructive-interference-size=
+Common Joined UInteger Var(param_destruct_interfere_size) Init(0) Param Optimization
+The minimum recommended offset between two concurrently-accessed objects to
+avoid additional performance degradation due to contention introduced by the
+implementation.  Typically the L1 cache line size, but can be larger to
+accommodate a variety of target processors with different cache line sizes.
+C++17 code might use this value in structure layout, but is strongly
+discouraged from doing so in public ABIs.
+
+-param=constructive-interference-size=
+Common Joined UInteger Var(param_construct_interfere_size) Init(0) Param Optimization
+The maximum recommended size of contiguous memory occupied by two objects
+accessed with temporal locality by concurrent threads.  Typically the L1 cache
+line size, but can be smaller to accommodate a variety of target processors with
+different cache line sizes.
+
 -param=l1-cache-size=
 Common Joined UInteger Var(param_l1_cache_size) Init(64) Param Optimization
 The size of L1 cache.
diff --git a/gcc/testsuite/g++.dg/warn/Winterference-2.C b/gcc/testsuite/g++.dg/warn/Winterference-2.C
new file mode 100644
index 0000000..2af75c6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Winterference-2.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++20 } }
+// { dg-additional-options -fmodules-ts }
+
+module ;
+
+#include <new>
+
+export module foo;
+
+export {
+  struct A {
+    alignas(std::hardware_destructive_interference_size) int x; // { dg-warning Winterference-size }
+  };
+}
diff --git a/gcc/testsuite/g++.dg/warn/Winterference.C b/gcc/testsuite/g++.dg/warn/Winterference.C
new file mode 100644
index 0000000..57c001b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Winterference.C
@@ -0,0 +1,6 @@
+// Test that we warn about use of std::hardware_destructive_interference_size
+// in a header.
+// { dg-do compile { target c++17 } }
+
+// { dg-warning Winterference-size "" { target *-*-* } 0 }
+#include "Winterference.H"
diff --git a/gcc/testsuite/g++.dg/warn/Winterference.H b/gcc/testsuite/g++.dg/warn/Winterference.H
new file mode 100644
index 0000000..36f0ad5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Winterference.H
@@ -0,0 +1,7 @@
+#include <new>
+
+struct A
+{
+  alignas(std::hardware_destructive_interference_size) int i;
+  alignas(std::hardware_destructive_interference_size) int j;
+};
diff --git a/gcc/testsuite/g++.target/aarch64/interference.C b/gcc/testsuite/g++.target/aarch64/interference.C
new file mode 100644
index 0000000..0fc0165
--- /dev/null
+++ b/gcc/testsuite/g++.target/aarch64/interference.C
@@ -0,0 +1,9 @@
+// Test C++17 hardware interference size constants
+// { dg-do compile { target c++17 } }
+
+#include <new>
+
+// Most AArch64 CPUs have an L1 cache line size of 64, but some recent ones use
+// 128 or even 256.
+static_assert(std::hardware_destructive_interference_size == 256);
+static_assert(std::hardware_constructive_interference_size == 64);
diff --git a/gcc/testsuite/g++.target/arm/interference.C b/gcc/testsuite/g++.target/arm/interference.C
new file mode 100644
index 0000000..34fe8a5
--- /dev/null
+++ b/gcc/testsuite/g++.target/arm/interference.C
@@ -0,0 +1,9 @@
+// Test C++17 hardware interference size constants
+// { dg-do compile { target c++17 } }
+
+#include <new>
+
+// Recent ARM CPUs have a cache line size of 64.  Older ones have
+// a size of 32, but I guess they're old enough that we don't care?
+static_assert(std::hardware_destructive_interference_size == 64);
+static_assert(std::hardware_constructive_interference_size == 64);
diff --git a/gcc/testsuite/g++.target/i386/interference.C b/gcc/testsuite/g++.target/i386/interference.C
new file mode 100644
index 0000000..c7b910e
--- /dev/null
+++ b/gcc/testsuite/g++.target/i386/interference.C
@@ -0,0 +1,8 @@
+// Test C++17 hardware interference size constants
+// { dg-do compile { target c++17 } }
+
+#include <new>
+
+// It is generally agreed that these are the right values for all x86.
+static_assert(std::hardware_destructive_interference_size == 64);
+static_assert(std::hardware_constructive_interference_size == 64);
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index f950bf0..f41004b 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -140,6 +140,9 @@
 #define __cpp_lib_filesystem 201703
 #define __cpp_lib_gcd 201606
 #define __cpp_lib_gcd_lcm 201606
+#ifdef __GCC_DESTRUCTIVE_SIZE
+# define __cpp_lib_hardware_interference_size 201703L
+#endif
 #define __cpp_lib_hypot 201603
 #define __cpp_lib_invoke 201411L
 #define __cpp_lib_lcm 201606
diff --git a/libstdc++-v3/libsupc++/new b/libstdc++-v3/libsupc++/new
index 3349b13..7bc67a6 100644
--- a/libstdc++-v3/libsupc++/new
+++ b/libstdc++-v3/libsupc++/new
@@ -183,9 +183,9 @@ inline void operator delete[](void*, void*) _GLIBCXX_USE_NOEXCEPT { }
 } // extern "C++"
 
 #if __cplusplus >= 201703L
-#ifdef _GLIBCXX_HAVE_BUILTIN_LAUNDER
 namespace std
 {
+#ifdef _GLIBCXX_HAVE_BUILTIN_LAUNDER
 #define __cpp_lib_launder 201606
   /// Pointer optimization barrier [ptr.launder]
   template<typename _Tp>
@@ -205,8 +205,14 @@ namespace std
   void launder(const void*) = delete;
   void launder(volatile void*) = delete;
   void launder(const volatile void*) = delete;
-}
 #endif // _GLIBCXX_HAVE_BUILTIN_LAUNDER
+
+#ifdef __GCC_DESTRUCTIVE_SIZE
+# define __cpp_lib_hardware_interference_size 201703L
+  inline constexpr size_t hardware_destructive_interference_size = __GCC_DESTRUCTIVE_SIZE;
+  inline constexpr size_t hardware_constructive_interference_size = __GCC_CONSTRUCTIVE_SIZE;
+#endif // __GCC_DESTRUCTIVE_SIZE
+}
 #endif // C++17
 
 #if __cplusplus > 201703L