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bug#65491: [PATCH] Improve performance allocating vectors
From: |
Mattias Engdegård |
Subject: |
bug#65491: [PATCH] Improve performance allocating vectors |
Date: |
Sat, 16 Sep 2023 19:03:33 +0200 |
16 sep. 2023 kl. 18.54 skrev Eli Zaretskii <eliz@gnu.org>:
> It does, but LISP_WORD_TAG(type) is a 64=bit type with the only bits
> set above 32 bit, so how casting it to uintptr_t is TRT?
Because XUNTAG is used to get the pointer part; we don't want the tag bits.
> Why did you need to change the original cast in the first place?
The commit message tried to explain that, but in essence, the old code untagged
a Lisp_Object value by casting it to char *, then do pointer arithmetic on
that, and then cast the result to whatever pointer we want.
The C standard severely restricts pointer arithmetic: in particular, for P+X
where X is an integer and P is a pointer, P cannot be null (nor will P+X, since
both P and P+X must be pointers to objects in the same array).
This means that XUNTAG could never reliably untag a null pointer and this did
cause mayhem in some places. We have just been lucky not to trigger it so far
but I noticed when attempting to make some innocent-looking changes.
- bug#65491: [PATCH] Improve performance allocating vectors, Mattias Engdegård, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Eli Zaretskii, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Eli Zaretskii, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Mattias Engdegård, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Eli Zaretskii, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors,
Mattias Engdegård <=
- bug#65491: [PATCH] Improve performance allocating vectors, Eli Zaretskii, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Po Lu, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Stefan Monnier, 2023/09/17
- bug#65491: [PATCH] Improve performance allocating vectors, Po Lu, 2023/09/17
- bug#65491: [PATCH] Improve performance allocating vectors, Stefan Monnier, 2023/09/17
- bug#65491: [PATCH] Improve performance allocating vectors, Po Lu, 2023/09/17
- bug#65491: [PATCH] Improve performance allocating vectors, Stefan Monnier, 2023/09/18
- bug#65491: [PATCH] Improve performance allocating vectors, Mattias Engdegård, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Eli Zaretskii, 2023/09/16
- bug#65491: [PATCH] Improve performance allocating vectors, Mattias Engdegård, 2023/09/16