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compiler: for package-scope "a = b; b = x" just set "a = x"

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This avoids requiring an init function to initialize the variable.
This can only be done if x is a static initializer.

The go1.15rc1 runtime package relies on this optimization.
The package has a variable "var maxSearchAddr = maxOffAddr".
The maxSearchAddr variable is used by code that runs before package
initialization is complete.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/245098
This commit is contained in:
Ian Lance Taylor 2020-07-27 14:22:55 -07:00
parent 1b024c9a29
commit e1d3a86fd3
2 changed files with 31 additions and 8 deletions
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2
gcc/go/gofrontend/MERGE
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@ -1,4 +1,4 @@
e86f2cb5d6b1984fde345d6ade605e377fa38c04
8b9c7fb00ccaf1d4bcc8d581a1a4d46a35771b77
The first line of this file holds the git revision number of the last
merge done from the gofrontend repository.

37
gcc/go/gofrontend/gogo.cc
View File

@ -1622,16 +1622,31 @@ Gogo::write_globals()
// The initializer is constant if it is the zero-value of the
// variable's type or if the initial value is an immutable value
// that is not copied to the heap.
bool is_static_initializer = false;
if (var->init() == NULL)
Expression* init = var->init();
// If we see "a = b; b = x", and x is a static
// initializer, just set a to x.
while (init != NULL && init->var_expression() != NULL)
{
Named_object* ino = init->var_expression()->named_object();
if (!ino->is_variable() || ino->package() != NULL)
break;
Expression* ino_init = ino->var_value()->init();
if (ino->var_value()->has_pre_init()
|| ino_init == NULL
|| !ino_init->is_static_initializer())
break;
init = ino_init;
}
bool is_static_initializer;
if (init == NULL)
is_static_initializer = true;
else
{
Type* var_type = var->type();
Expression* init = var->init();
Expression* init_cast =
Expression::make_cast(var_type, init, var->location());
is_static_initializer = init_cast->is_static_initializer();
init = Expression::make_cast(var_type, init, var->location());
is_static_initializer = init->is_static_initializer();
}
// Non-constant variable initializations might need to create
@ -1650,7 +1665,15 @@ Gogo::write_globals()
}
var_init_fn = init_fndecl;
}
Bexpression* var_binit = var->get_init(this, var_init_fn);
Bexpression* var_binit;
if (init == NULL)
var_binit = NULL;
else
{
Translate_context context(this, var_init_fn, NULL, NULL);
var_binit = init->get_backend(&context);
}
if (var_binit == NULL)
;