https://codereview.chromium.org/164793003/diff/1/src/a64/macro-assembler-a64.cc
File src/a64/macro-assembler-a64.cc (right):
https://codereview.chromium.org/164793003/diff/1/src/a64/macro-assembler-a64.cc#newcode5172
src/a64/macro-assembler-a64.cc:5172: void
UseScratchRegisterScope::Include(const CPURegList& regs) {
They're useful in tests (such as Dump), but there aren't many cases
where we
need to explicitly exclude registers. Most of the time, if we need to
use a
specific register, it's because we're calling a function. We _could_
go
over-the-top and Exclude the arguments registers (x0, ...) in those
cases, to be
safe, but such an invasive change is almost certainly not worthwhile.
Regarding incorrect use: It is true that modifications to TmpList()
will be
reverted when a scope exits, even if a caller started the scope, but
we thought
that was the correct behaviour. What accidental use-case did you think
of?
The accidental use-case I'm thinking of is something along the lines of
code which adds a random register to a scratch register scope, does some
work, then accidentally uses that same register thinking it's free
(either within the same scope, or a further scope of a called
MacroAssembler method). E.g.:
MacroAssembler::bar(... Register scratch) {
// load scratch1
Mov(x1, MemOperand...) // clobbers scratch1
// use scratch1
}
MacroAssembler::foo() {
UseScratchRegisterScope tmps(this);
tmps.Include(scratch1);
// ...
bar(..., scratch1); // mistakenly think scratch1 is free
}
I realise the use of scratch1 after including it in the
UseScratchRegisterScope in foo is incorrect, but I could see something
like this easily being accidentally introduced via copy/pasted code or
in extra long scopes where it's not obvious which temps are available.
Also, even if I remove Include and Exclude, the same behaviour could
be achieved
by directly manipulating TmpList(), but I don't want to encourage
that.
Sure anything's possible by manipulating TmpList, but as you say we
don't want to encourage this, and Include/Exclude encourage this
manipulation of the available scratch registers :).
https://codereview.chromium.org/164793003/
--
--
v8-dev mailing list
[email protected]
http://groups.google.com/group/v8-dev
---
You received this message because you are subscribed to the Google Groups "v8-dev" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
For more options, visit https://groups.google.com/groups/opt_out.
