Hello!
We can implement atomic_fetch_sub with atomic_fetch_add and inverted
operand 2. However, we have to prevent overflows with negation, so
only const_int operand 2 is allowed in the expander.
2012-08-02 Uros Bizjak <ubiz...@gmail.com>
PR target/54087
* config/i386/sync.md (atomic_fetch_sub<mode>): New expander.
Tested on x86_64-pc-linux-gnu {,-m32}.
I will wait for a day for possible comments.
Uros.
Index: config/i386/sync.md
===================================================================
--- config/i386/sync.md (revision 190111)
+++ config/i386/sync.md (working copy)
@@ -520,6 +520,31 @@
return "lock{%;} add{<imodesuffix>}\t{%1, %0|%0, %1}";
})
+;; We can use atomic_fetch_add with negated operand 2 to implement
+;; atomic_fetch_sub. We have to prevent overflows with negation, so
+;; only const_int operand 2 is allowed.
+(define_expand "atomic_fetch_sub<mode>"
+ [(set (match_operand:SWI 0 "register_operand")
+ (unspec_volatile:SWI
+ [(match_operand:SWI 1 "memory_operand")
+ (match_operand:SI 3 "const_int_operand")] ;; model
+ UNSPECV_XCHG))
+ (set (match_dup 1)
+ (minus:SWI (match_dup 1)
+ (match_operand:SWI 2 "const_int_operand")))
+ (clobber (reg:CC FLAGS_REG))]
+ "TARGET_XADD"
+{
+ /* Avoid overflows. */
+ if (mode_signbit_p (<MODE>mode, operands[2]))
+ FAIL;
+
+ emit_insn (gen_atomic_fetch_add<mode> (operands[0], operands[1],
+ negate_rtx (<MODE>mode, operands[2]),
+ operands[3]));
+ DONE;
+})
+
;; Recall that xchg implicitly sets LOCK#, so adding it again wastes space.
;; In addition, it is always a full barrier, so we can ignore the memory model.
(define_insn "atomic_exchange<mode>"
