On 6/20/24 7:41 PM, Warner Losh wrote:
The branch main has been updated by imp:
URL:
https://cgit.FreeBSD.org/src/commit/?id=2508372b7b46117a9fb801b50624265d30888442
commit 2508372b7b46117a9fb801b50624265d30888442
Author: Warner Losh <[email protected]>
AuthorDate: 2024-06-20 23:02:42 +0000
Commit: Warner Losh <[email protected]>
CommitDate: 2024-06-21 02:41:08 +0000
cdefs.h: Assume the compiler supports at least GNU C 3.0 extensions
All compilers that can build FreeBSD binaries (as opposed to the entire
system) support at least gcc 9 (gcc, clang, tcc). Even pcc supports most
of the gcc 4.3 attributes. Make this file simpler by removing support
for pre-3.0 compilers.
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D45653
Sponsored by: Netflix
---
sys/sys/cdefs.h | 79 +--------------------------------------------------------
1 file changed, 1 insertion(+), 78 deletions(-)
diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 476c89d1dddb..88019819eb35 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -389,45 +367,10 @@
*/
#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901
#define __restrict restrict
-#elif !__GNUC_PREREQ__(2, 95)
-#define __restrict
#endif
-/*
- * GNU C version 2.96 adds explicit branch prediction so that
- * the CPU back-end can hint the processor and also so that
- * code blocks can be reordered such that the predicted path
- * sees a more linear flow, thus improving cache behavior, etc.
- *
- * The following two macros provide us with a way to utilize this
- * compiler feature. Use __predict_true() if you expect the expression
- * to evaluate to true, and __predict_false() if you expect the
- * expression to evaluate to false.
- *
- * A few notes about usage:
- *
- * * Generally, __predict_false() error condition checks (unless
- * you have some _strong_ reason to do otherwise, in which case
- * document it), and/or __predict_true() `no-error' condition
- * checks, assuming you want to optimize for the no-error case.
- *
- * * Other than that, if you don't know the likelihood of a test
- * succeeding from empirical or other `hard' evidence, don't
- * make predictions.
- *
- * * These are meant to be used in places that are run `a lot'.
- * It is wasteful to make predictions in code that is run
- * seldomly (e.g. at subsystem initialization time) as the
- * basic block reordering that this affects can often generate
- * larger code.
- */
-#if __GNUC_PREREQ__(2, 96)
#define __predict_true(exp) __builtin_expect((exp), 1)
#define __predict_false(exp) __builtin_expect((exp), 0)
-#else
-#define __predict_true(exp) (exp)
-#define __predict_false(exp) (exp)
-#endif
I think the comment was worth keeping around. You just need to modify
the start of it to "Modern compilers include explicit branch prediction..."
In particular, I think our current practice is still to apply prediction
hints rather conservatively.
--
John Baldwin
