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Re: [Emacs-diffs] feature/byte-switch 086c4ea: * src/bytecode.c: (exec_b

From: Vibhav Pant
Subject: Re: [Emacs-diffs] feature/byte-switch 086c4ea: * src/bytecode.c: (exec_byte_code) Use hash_lookup for Bswitch
Date: Thu, 19 Jan 2017 17:53:50 +0000

I see. For now, I've made the type check only work when BYTE_CODE_SAFE is defined at compile time. I'll make it a part of the regular code once the byte-switch is implemented and there are no performance issues with doing so.

On Thu, 19 Jan 2017, 22:04 Stefan Monnier, <address@hidden> wrote:
> On another thought, `byte-switch` is used while compiling certain
> `cond` forms. It
> replaces the traditional goto-if-nil bytecode by using a hash table mapping
> values to addresses/tags to be jumped to. Since `byte-switch` is essentially a
> "dynamic" goto (in the sense that the address/tag cannot be known at
> compile time),
> wouldn't doing a runtime type check in the bytecode VM for what is  a
> hash table lookup + goto have a significant performance penalty?

I don't know.  Only measurement can tell.  My guess is that hash_lookup
already takes a significant amount of time, so a HASH_TABLE_P test would be
negligible in comparison.


> On Wed, Jan 18, 2017 at 11:18 PM, Stefan Monnier
> <address@hidden> wrote:
>>> * src/bytecode.c: (exec_byte_code) Use hash_lookup for Bswitch
>>> Fgethash type checks the provided table object, which is unnecessary
>>> for compiled bytecode.
>> While it's true that we can cause a core dump of Emacs if we feed it an
>> invalid .elc file, that's a "feature" I'd rather shrink rather
>> than generalize.
>> Stefan

> --
> Vibhav Pant
> address@hidden

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