Re: [PATCH] PR libstdc++/86846 Alternative to pointer-width atomics

[Jonathan Wakely]

On 15/08/18 10:40 +0100, Szabolcs Nagy wrote:

On 14/08/18 14:13, Jonathan Wakely wrote:

Define a class using std::mutex for when std::atomic<memory_resource*>
cannot be used to implement the default memory resource.

When std::mutex constructor is not constexpr the constant_init trick
won't work, so just define a global and use init_priority for it. The
compiler warns about using reserved priority, so put the definition in a
header file using #pragma GCC system_header to suppress the warning.

    PR libstdc++/86846
    * src/c++17/default_resource.h: New file, defining default_res.
    * src/c++17/memory_resource.cc [ATOMIC_POINTER_LOCK_FREE != 2]
    (atomic_mem_res): Define alternative for atomic<memory_resource*>
    using a mutex instead of atomics.

Tested x86_64-linux, committed to trunk.

build fails with arm-none-eabi target:

Making all in c++17
make[4]: Entering directory '/B/arm-none-eabi/libstdc++-v3/src/c++17'

/bin/sh ../../libtool --tag CXX --tag disable-shared --mode=compile /B/./gcc/xgcc -shared-libgcc -B/B/./gcc -nostdinc++ -L/B/arm-none-eabi/libstdc++-v3/src -L/B/arm-none-eabi/libstdc++-v3/src/.libs -L/B/arm-none-eabi/libstdc++-v3/libsupc++/.libs -B/P/arm-none-eabi/bin/ -B/P/arm-none-eabi/lib/ -isystem /P/arm-none-eabi/include -isystem /P/arm-none-eabi/sys-include -I/S/libstdc++-v3/../libgcc -I/B/arm-none-eabi/libstdc++-v3/include/arm-none-eabi -I/B/arm-none-eabi/libstdc++-v3/include -I/S/libstdc++-v3/libsupc++ -std=gnu++17 -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi=2 -fdiagnostics-show-location=once -ffunction-sections -fdata-sections -frandom-seed=memory_resource.lo -g -ffunction-sections -fdata-sections -O2 -c -o memory_resource.lo /S/libstdc++-v3/src/c++17/memory_resource.cc libtool: compile: /B/./gcc/xgcc -shared-libgcc -B/B/./gcc -nostdinc++ -L/B/arm-none-eabi/libstdc++-v3/src -L/B/arm-none-eabi/libstdc++-v3/src/.libs -L/B/arm-none-eabi/libstdc++-v3/libsupc++/.libs -B/P/arm-none-eabi/bin/ -B/P/arm-none-eabi/lib/ -isystem /P/arm-none-eabi/include -isystem /P/arm-none-eabi/sys-include -I/S/libstdc++-v3/../libgcc -I/B/arm-none-eabi/libstdc++-v3/include/arm-none-eabi -I/B/arm-none-eabi/libstdc++-v3/include -I/S/libstdc++-v3/libsupc++ -std=gnu++17 -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi=2 -fdiagnostics-show-location=once -ffunction-sections -fdata-sections -frandom-seed=memory_resource.lo -g -ffunction-sections -fdata-sections -O2 -c /S/libstdc++-v3/src/c++17/memory_resource.cc -o memory_resource.o

/S/libstdc++-v3/src/c++17/memory_resource.cc:102:7: error: 'mutex' does not 
name a type
102 |       mutex mx;

Sorry about that, it should be fixed at r263554 by the commit of this
patch.





commit 21cebbbedfbbe8fc5c4b1ebc59bef1b8f090bc6b
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Aug 15 10:55:31 2018 +0100

    Fix single-threaded build for targets without atomics
    
            * src/c++17/memory_resource.cc [!_GLIBCXX_HAS_GTHREADS]
            (atomic_mem_res): Add unsynchronized definition for single-threaded.

diff --git a/libstdc++-v3/src/c++17/memory_resource.cc b/libstdc++-v3/src/c++17/memory_resource.cc
index bd8f32d931e..aa82813e645 100644
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/libstdc++-v3/src/c++17/memory_resource.cc
@@ -88,7 +88,7 @@ namespace pmr
 #if ATOMIC_POINTER_LOCK_FREE == 2
     using atomic_mem_res = atomic<memory_resource*>;
 # define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
-#else
+#elif defined(_GLIBCXX_HAS_GTHREADS)
     // Can't use pointer-width atomics, define a type using a mutex instead:
     struct atomic_mem_res
     {
@@ -114,6 +114,26 @@ namespace pmr
 	return std::exchange(val, r);
       }
     };
+#else
+# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
+    // Single-threaded, no need for synchronization
+    struct atomic_mem_res
+    {
+      constexpr
+      atomic_mem_res(memory_resource* r) : val(r) { }
+
+      memory_resource* val;
+
+      memory_resource* load() const
+      {
+	return val;
+      }
+
+      memory_resource* exchange(memory_resource* r)
+      {
+	return std::exchange(val, r);
+      }
+    };
 #endif // ATOMIC_POINTER_LOCK_FREE == 2
 
 #ifdef _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED