c45eaeabb7
Summary: [Clang] Attribute to allow defining undef global variables Initializing global variables is very cheap on hosted implementations. The C semantics of zero initializing globals work very well there. It is not necessarily cheap on freestanding implementations. Where there is no loader available, code must be emitted near the start point to write the appropriate values into memory. At present, external variables can be declared in C++ and definitions provided in assembly (or IR) to achive this effect. This patch provides an attribute in order to remove this reason for writing assembly for performance sensitive freestanding implementations. A close analogue in tree is LDS memory for amdgcn, where the kernel is responsible for initializing the memory after it starts executing on the gpu. Uninitalized variables in LDS are observably cheaper than zero initialized. Patch is loosely based on the cuda __shared__ and opencl __local variable implementation which also produces undef global variables. Reviewers: kcc, rjmccall, rsmith, glider, vitalybuka, pcc, eugenis, vlad.tsyrklevich, jdoerfert, gregrodgers, jfb, aaron.ballman Reviewed By: rjmccall, aaron.ballman Subscribers: Anastasia, aaron.ballman, davidb, Quuxplusone, dexonsmith, cfe-commits Tags: #clang Differential Revision: https://reviews.llvm.org/D74361
61 lines
2.5 KiB
C++
61 lines
2.5 KiB
C++
// RUN: %clang_cc1 %s -verify -fsyntax-only
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int good __attribute__((loader_uninitialized));
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static int local_ok __attribute__((loader_uninitialized));
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int hidden_ok __attribute__((visibility("hidden"))) __attribute__((loader_uninitialized));
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const int still_cant_be_const __attribute__((loader_uninitialized));
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// expected-error@-1 {{default initialization of an object of const type}}
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extern int external_rejected __attribute__((loader_uninitialized));
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// expected-error@-1 {{variable 'external_rejected' cannot be declared both 'extern' and with the 'loader_uninitialized' attribute}}
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int noargs __attribute__((loader_uninitialized(0)));
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// expected-error@-1 {{'loader_uninitialized' attribute takes no arguments}}
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int init_rejected __attribute__((loader_uninitialized)) = 42;
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// expected-error@-1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}}
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void func() __attribute__((loader_uninitialized))
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// expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}}
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{
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int local __attribute__((loader_uninitialized));
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// expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}}
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static int sl __attribute__((loader_uninitialized));
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}
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struct s {
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__attribute__((loader_uninitialized)) int field;
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// expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}}
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static __attribute__((loader_uninitialized)) int sfield;
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} __attribute__((loader_uninitialized));
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// expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}}
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int redef_attr_first __attribute__((loader_uninitialized));
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int redef_attr_first;
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// expected-error@-1 {{redefinition of 'redef_attr_first'}}
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// expected-note@-3 {{previous definition is here}}
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int redef_attr_second;
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int redef_attr_second __attribute__((loader_uninitialized));
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// expected-warning@-1 {{attribute declaration must precede definition}}
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// expected-note@-3 {{previous definition is here}}
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// expected-error@-3 {{redefinition of 'redef_attr_second'}}
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// expected-note@-5 {{previous definition is here}}
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struct trivial {};
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trivial default_ok __attribute__((loader_uninitialized));
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trivial value_rejected __attribute__((loader_uninitialized)) {};
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// expected-error@-1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}}
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struct nontrivial
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{
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nontrivial() {}
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};
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nontrivial needs_trivial_ctor __attribute__((loader_uninitialized));
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// expected-error@-1 {{variable with 'loader_uninitialized' attribute must have a trivial default constructor}}
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