[v1,2/2] x86_64: Add {u}int_fast defs ideal for x86_64
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| dj/TryBot-apply_patch |
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Patch applied to master at the time it was sent
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| dj/TryBot-32bit |
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Build for i686
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Commit Message
{u}int_fast{16|32} should never use 64-bit types. 64-bit instructions
often use more code size (REX prefix) and a variety of instructions are
slower on many implementations (div, bswap, etc...).
Full xcheck passes on x86_64.
---
sysdeps/x86_64/int-fast.h | 50 +++++++++++++++++++++++++++++++++++++++
1 file changed, 50 insertions(+)
create mode 100644 sysdeps/x86_64/int-fast.h
Comments
Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org> writes:
> {u}int_fast{16|32} should never use 64-bit types. 64-bit instructions
> often use more code size (REX prefix) and a variety of instructions
> are slower on many implementations (div, bswap, etc...).
Isn't that going to cause ABI breakage in other libraries, though?
For example, the jasper JPEG2000 decoder library uses stdint.h's
uint_fast32_t in its public API, and the HDF5 scientific data library
includes constants for the sizes of various types including *int*fast*.
Thanks,
* Adam Sampson via Libc-alpha:
> Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org> writes:
>
>> {u}int_fast{16|32} should never use 64-bit types. 64-bit instructions
>> often use more code size (REX prefix) and a variety of instructions
>> are slower on many implementations (div, bswap, etc...).
>
> Isn't that going to cause ABI breakage in other libraries, though?
> For example, the jasper JPEG2000 decoder library uses stdint.h's
> uint_fast32_t in its public API, and the HDF5 scientific data library
> includes constants for the sizes of various types including *int*fast*.
Agreed, it's too late for such changes.
Thanks,
Florian
On Sun, 10 Apr 2022, Noah Goldstein via Libc-alpha wrote:
> {u}int_fast{16|32} should never use 64-bit types. 64-bit instructions
> often use more code size (REX prefix) and a variety of instructions are
> slower on many implementations (div, bswap, etc...).
Apart from the other issues discussed with this patch, installed headers
should not be specific to either x86_64 or 32-bit x86; such headers, where
architecture-specific, should go in an x86/ sysdeps directory and contain
appropriate conditionals within the header, so a single set of installed
headers works with a multilibbed compiler.
> +# include <sysdeps/generic/int-fast.h>
And an installed header can't do that sort of thing, because only a single
version of any such header gets installed. If you want to include a
generic file it has to have a different name, with both being installed
(cf. bits/mman-*.h, for example).
On Thu, Apr 14, 2022 at 5:24 PM Joseph Myers <joseph@codesourcery.com> wrote:
>
> On Sun, 10 Apr 2022, Noah Goldstein via Libc-alpha wrote:
>
> > {u}int_fast{16|32} should never use 64-bit types. 64-bit instructions
> > often use more code size (REX prefix) and a variety of instructions are
> > slower on many implementations (div, bswap, etc...).
>
> Apart from the other issues discussed with this patch, installed headers
> should not be specific to either x86_64 or 32-bit x86; such headers, where
> architecture-specific, should go in an x86/ sysdeps directory and contain
> appropriate conditionals within the header, so a single set of installed
> headers works with a multilibbed compiler.
>
> > +# include <sysdeps/generic/int-fast.h>
>
> And an installed header can't do that sort of thing, because only a single
> version of any such header gets installed. If you want to include a
> generic file it has to have a different name, with both being installed
> (cf. bits/mman-*.h, for example).
Good info to have but the patches have been drop for the ABI concerns.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com
new file mode 100644
@@ -0,0 +1,50 @@
+/* Copyright (C) 2022 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#ifndef _INT_FAST_X86_64_H
+#define _INT_FAST_X86_64_H 1
+
+#define __HAS_INT_FAST_DEFS 1
+
+/* Signed. */
+typedef signed char int_fast8_t;
+/* On x86_64 32-bit instructions are almost always fastest. */
+typedef int int_fast16_t;
+typedef int int_fast32_t;
+typedef long int int_fast64_t;
+
+/* Unsigned. */
+typedef unsigned char uint_fast8_t;
+/* On x86_64 32-bit instructions are almost always fastest. */
+typedef unsigned int uint_fast16_t;
+typedef unsigned int uint_fast32_t;
+typedef unsigned long int uint_fast64_t;
+
+
+# define __INT_FAST8_BASE INT8
+# define __INT_FAST16_BASE INT32
+# define __INT_FAST32_BASE INT32
+# define __INT_FAST64_BASE INT64
+
+# define __UINT_FAST8_BASE UINT8
+# define __UINT_FAST16_BASE UINT32
+# define __UINT_FAST32_BASE UINT32
+# define __UINT_FAST64_BASE UINT64
+
+# include <sysdeps/generic/int-fast.h>
+
+#endif