Author: rlibby
Date: Fri Feb 28 18:32:36 2020
New Revision: 358439
URL: https://svnweb.freebsd.org/changeset/base/358439
Log:
amd64 atomic.h: minor codegen optimization in flag access
Previously the pattern to extract status flags from inline assembly
blocks was to use setcc in the block to write the flag to a register.
This was suboptimal in a few ways:
- It would lead to code like: sete %cl; test %cl; jne, i.e. a flag
would just be loaded into a register and then reloaded to a flag.
- The setcc would force the block to use an additional register.
- If the client code didn't care for the flag value then the setcc
would be entirely pointless but could not be eliminated by the
optimizer.
A more modern inline asm construct (since gcc 6 and clang 9) allows for
"flag output operands", where a C variable can be written directly from
a flag. The optimizer can then use this to produce direct code where
the flag does not take a trip through a register.
In practice this makes each affected operation sequence shorter by five
bytes of instructions. It's unlikely this has a measurable performance
impact.
Reviewed by: kib, markj, mjg
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23869
Modified:
head/sys/amd64/include/atomic.h
Modified: head/sys/amd64/include/atomic.h
==============================================================================
--- head/sys/amd64/include/atomic.h Fri Feb 28 17:41:46 2020
(r358438)
+++ head/sys/amd64/include/atomic.h Fri Feb 28 18:32:36 2020
(r358439)
@@ -201,9 +201,8 @@ atomic_cmpset_##TYPE(volatile u_##TYPE *dst, u_##TYPE
__asm __volatile( \
" " MPLOCKED " " \
" cmpxchg %3,%1 ; " \
- " sete %0 ; " \
"# atomic_cmpset_" #TYPE " " \
- : "=q" (res), /* 0 */ \
+ : "=@cce" (res), /* 0 */ \
"+m" (*dst), /* 1 */ \
"+a" (expect) /* 2 */ \
: "r" (src) /* 3 */ \
@@ -219,9 +218,8 @@ atomic_fcmpset_##TYPE(volatile u_##TYPE *dst, u_##TYPE
__asm __volatile( \
" " MPLOCKED " " \
" cmpxchg %3,%1 ; " \
- " sete %0 ; " \
"# atomic_fcmpset_" #TYPE " " \
- : "=q" (res), /* 0 */ \
+ : "=@cce" (res), /* 0 */ \
"+m" (*dst), /* 1 */ \
"+a" (*expect) /* 2 */ \
: "r" (src) /* 3 */ \
@@ -278,9 +276,8 @@ atomic_testandset_int(volatile u_int *p, u_int v)
__asm __volatile(
" " MPLOCKED " "
" btsl %2,%1 ; "
- " setc %0 ; "
"# atomic_testandset_int"
- : "=q" (res), /* 0 */
+ : "=@ccc" (res), /* 0 */
"+m" (*p) /* 1 */
: "Ir" (v & 0x1f) /* 2 */
: "cc");
@@ -295,9 +292,8 @@ atomic_testandset_long(volatile u_long *p, u_int v)
__asm __volatile(
" " MPLOCKED " "
" btsq %2,%1 ; "
- " setc %0 ; "
"# atomic_testandset_long"
- : "=q" (res), /* 0 */
+ : "=@ccc" (res), /* 0 */
"+m" (*p) /* 1 */
: "Jr" ((u_long)(v & 0x3f)) /* 2 */
: "cc");
@@ -312,9 +308,8 @@ atomic_testandclear_int(volatile u_int *p, u_int v)
__asm __volatile(
" " MPLOCKED " "
" btrl %2,%1 ; "
- " setc %0 ; "
"# atomic_testandclear_int"
- : "=q" (res), /* 0 */
+ : "=@ccc" (res), /* 0 */
"+m" (*p) /* 1 */
: "Ir" (v & 0x1f) /* 2 */
: "cc");
@@ -329,9 +324,8 @@ atomic_testandclear_long(volatile u_long *p, u_int v)
__asm __volatile(
" " MPLOCKED " "
" btrq %2,%1 ; "
- " setc %0 ; "
"# atomic_testandclear_long"
- : "=q" (res), /* 0 */
+ : "=@ccc" (res), /* 0 */
"+m" (*p) /* 1 */
: "Jr" ((u_long)(v & 0x3f)) /* 2 */
: "cc");
_______________________________________________
svn-src-head@freebsd.org mailing list
https://lists.freebsd.org/mailman/listinfo/svn-src-head
To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
