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compiler: fix evaluation order of LHS index expressions

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The spec says that when an index expression appears on the left hand
    side of an assignment, the operands should be evaluated. The
    gofrontend code was assuming that that only referred to the index
    operand. But discussion of https://golang.org/issue/23188 has
    clarified that this means both the slice/map/string operand and the
    index operand. Adjust the gofrontend code accordingly.
    
    Fixes golang/go#23188
    
    Reviewed-on: https://go-review.googlesource.com/123155

From-SVN: r262554
This commit is contained in:
Ian Lance Taylor 2018-07-11 14:22:12 +00:00
parent 2bae8b2fad
commit 305130b92e
3 changed files with 23 additions and 21 deletions
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2
gcc/go/gofrontend/MERGE
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@ -1,4 +1,4 @@
8ad67a72a4fa59efffc891e73ecf10020e3c565d
ea7ac7784791dca517b6681a02c39c11bf136755
The first line of this file holds the git revision number of the last
merge done from the gofrontend repository.

14
gcc/go/gofrontend/expressions.cc
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@ -10898,6 +10898,20 @@ Array_index_expression::do_check_types(Gogo*)
}
}
// The subexpressions of an array index must be evaluated in order.
// If this is indexing into an array, rather than a slice, then only
// the index should be evaluated. Since this is called for values on
// the left hand side of an assigment, evaluating the array, meaning
// copying the array, will cause a different array to be modified.
bool
Array_index_expression::do_must_eval_subexpressions_in_order(
int* skip) const
{
*skip = this->array_->type()->is_slice_type() ? 0 : 1;
return true;
}
// Flatten array indexing by using temporary variables for slices and indexes.
Expression*

28
gcc/go/gofrontend/expressions.h
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@ -2771,12 +2771,10 @@ class Index_expression : public Parser_expression
this->location());
}
// This shouldn't be called--we don't know yet.
bool
do_must_eval_subexpressions_in_order(int* skip) const
{
*skip = 1;
return true;
}
do_must_eval_subexpressions_in_order(int*) const
{ go_unreachable(); }
void
do_dump_expression(Ast_dump_context*) const;
@ -2882,11 +2880,7 @@ class Array_index_expression : public Expression
}
bool
do_must_eval_subexpressions_in_order(int* skip) const
{
*skip = 1;
return true;
}
do_must_eval_subexpressions_in_order(int* skip) const;
bool
do_is_addressable() const;
@ -2965,11 +2959,8 @@ class String_index_expression : public Expression
}
bool
do_must_eval_subexpressions_in_order(int* skip) const
{
*skip = 1;
return true;
}
do_must_eval_subexpressions_in_order(int*) const
{ return true; }
Bexpression*
do_get_backend(Translate_context*);
@ -3052,11 +3043,8 @@ class Map_index_expression : public Expression
}
bool
do_must_eval_subexpressions_in_order(int* skip) const
{
*skip = 1;
return true;
}
do_must_eval_subexpressions_in_order(int*) const
{ return true; }
// A map index expression is an lvalue but it is not addressable.