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67 lines
2.7 KiB
67 lines
2.7 KiB
3 years ago
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c++: Optimize constinit thread_local vars [PR101786]
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The paper that introduced constinit mentioned in rationale that constinit
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can be used on externs as well and that it can be used to avoid the
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thread_local initialization wrappers, because the standard requires that
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if constinit is present on any declaration, it is also present on the
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initialization declaration, even if it is in some other TU etc.
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There is a small problem though, we use the tls wrappers not just if
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the thread_local variable needs dynamic initialization, but also when
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it has static initialization, but non-trivial destructor, as the
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"dynamic initialization" in that case needs to register the destructor.
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So, the following patch optimizes constinit thread_local vars only
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if we can prove they will not have non-trivial destructors. That includes
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the case where we have incomplete type where we don't know and need to
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conservatively assume the type will have non-trivial destructor at the
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initializing declaration side.
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2021-08-11 Jakub Jelinek <jakub@redhat.com>
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PR c++/101786
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* decl2.c (var_defined_without_dynamic_init): Return true for
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DECL_DECLARED_CONSTINIT_P with complete type and trivial destructor.
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* g++.dg/cpp2a/constinit16.C: New test.
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--- gcc/cp/decl2.c
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+++ gcc/cp/decl2.c
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@@ -3447,6 +3447,12 @@ set_guard (tree guard)
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static bool
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var_defined_without_dynamic_init (tree var)
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{
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+ /* constinit vars are guaranteed to not have dynamic initializer,
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+ but still registering the destructor counts as dynamic initialization. */
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+ if (DECL_DECLARED_CONSTINIT_P (var)
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+ && COMPLETE_TYPE_P (TREE_TYPE (var))
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+ && !TYPE_HAS_NONTRIVIAL_DESTRUCTOR (TREE_TYPE (var)))
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+ return true;
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/* If it's defined in another TU, we can't tell. */
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if (DECL_EXTERNAL (var))
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return false;
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--- gcc/testsuite/g++.dg/cpp2a/constinit16.C
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+++ gcc/testsuite/g++.dg/cpp2a/constinit16.C
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@@ -0,0 +1,21 @@
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+// PR c++/101786
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+// { dg-do compile { target c++20 } }
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+// { dg-add-options tls }
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+// { dg-require-alias "" }
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+// { dg-require-effective-target tls_runtime }
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+// { dg-final { scan-assembler-not "_ZTH17mythreadlocalvar1" } }
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+// { dg-final { scan-assembler "_ZTH17mythreadlocalvar2" } }
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+// { dg-final { scan-assembler-not "_ZTH17mythreadlocalvar3" } }
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+// { dg-final { scan-assembler "_ZTH17mythreadlocalvar4" } }
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+
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+extern thread_local constinit int mythreadlocalvar1;
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+struct S;
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+extern thread_local constinit S mythreadlocalvar2;
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+struct T { int t; };
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+extern thread_local constinit T mythreadlocalvar3;
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+struct U { int u; ~U (); };
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+extern thread_local constinit U mythreadlocalvar4;
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+int foo () { return mythreadlocalvar1; }
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+S *bar () { return &mythreadlocalvar2; }
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+T *baz () { return &mythreadlocalvar3; }
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+U *qux () { return &mythreadlocalvar4; }
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