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vm.c: answer str[0] from C in OP_GETIDX0 #7040

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Merged
matz merged 1 commit into from
Aug 9, 2026
Merged

vm.c: answer str[0] from C in OP_GETIDX0 #7040

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15 changes: 15 additions & 0 deletions src/vm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2143,6 +2143,21 @@ vm_op_getidx0(mrb_state *mrb, uint32_t a, uint16_t b, mrb_sym *midp)
}
return VM_NEXT;
}
else if (tt == MRB_TT_STRING) {
/* optimize only for String class; subclasses/singleton may override [] */
if (mrb_obj_ptr(recv)->c != mrb->string_class) goto getidx0_fallback;
{
/* mrb_str_aref() allocates, and an inline opcode never runs the cfunc
epilogue that would shrink the arena, so save and restore it here.
Unlike the Hash branch above, `ci` needs no refresh: this call cannot
run Ruby code and so cannot move the stack. */
int ai = mrb_gc_arena_save(mrb);
mrb_value val = mrb_str_aref(mrb, recv, mrb_fixnum_value(0), mrb_undef_value());
regs[a] = val;
mrb_gc_arena_restore(mrb, ai);
}
return VM_NEXT;
}
getidx0_fallback:
regs[a] = recv;
SET_FIXNUM_VALUE(regs[a+1], 0);
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12 changes: 12 additions & 0 deletions test/t/gc.rb
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Expand Up @@ -148,6 +148,18 @@
assert_operator GC.stat[:live] - base, :<, 5000
end

assert('OP_GETIDX0 does not retain a String result in the GC arena') do
s = "hello"
GC.start
base = GC.stat[:live]
i = 0
while i < 20000
s[0]
i += 1
end
assert_operator GC.stat[:live] - base, :<, 5000
end

assert('OP_GETIDX0 does not retain a Hash default in the GC arena') do
h = Hash.new { Object.new }
GC.start
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28 changes: 28 additions & 0 deletions test/t/string.rb
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Expand Up @@ -126,6 +126,34 @@
assert_equal 'xyz', k2
end

assert('String#[] redefined on String itself is bypassed by the index opcodes') do
# `OP_GETIDX` answers `s[1]` from C, and `OP_GETIDX0` answers `s[0]` the same
# way, whenever the receiver's class is exactly `String`. Both therefore
# bypass a redefinition installed on `String` itself, the same way the Array
# and Hash branches of those opcodes do. A subclass receiver fails the class
# guard and keeps reaching the redefinition.
String.class_eval do
alias_method :__aref_before_test, :[]
def [](*args)
:overridden
end
end
begin
s = 'hello'
sub = Class.new(String).new('hello')
assert_equal 'h', s[0]
assert_equal 'e', s[1]
assert_equal :overridden, sub[0]
ensure
String.class_eval do
alias_method :[], :__aref_before_test
# `remove_method` comes from mruby-metaprog, which the core test build
# does not have; the saved alias is harmless where it is missing.
remove_method :__aref_before_test if respond_to?(:remove_method, true)
end
end
end

assert('String#[]=') do
# length of args is 1
a = 'abc'
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