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[v2,04/10] RISC-V: Use STORE instead of AMOSWAP for atomic stores [PR 100265]

Message ID 20210505193651.2075405-5-cmuellner@gcc.gnu.org
State New
Headers show
Series Atomics improvements [PR100265/PR100266] | expand

Commit Message

Christoph Müllner May 5, 2021, 7:36 p.m. UTC 
Using AMOSWAP as atomic store does not allow us to do sub-word accesses.
Further, it is not consistent with our atomic_load () implementation.
The benefit of AMOSWAP is that the resulting code sequence will be
smaller (comapred to FENCE+STORE), however, this does not weight
out for the lack of sub-word accesses.
Additionally, HW implementors have claimed that an optimal
implementation AMOSWAP is slightly more expensive than FENCE+STORE.
So let's use STORE instead of AMOSWAP.

    gcc/
        PR 100265
        * config/riscv/sync.md (atomic_store<mode>):
          Remove.
---
 gcc/config/riscv/sync.md | 11 -----------
 1 file changed, 11 deletions(-)
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Patch

diff --git a/gcc/config/riscv/sync.md b/gcc/config/riscv/sync.md
index aeeb2e854b68..efd49745a8e2 100644
--- a/gcc/config/riscv/sync.md
+++ b/gcc/config/riscv/sync.md
@@ -57,17 +57,6 @@ 
 
 ;; Atomic memory operations.
 
-;; Implement atomic stores with amoswap.  Fall back to fences for atomic loads.
-(define_insn "atomic_store<mode>"
-  [(set (match_operand:GPR 0 "memory_operand" "=A")
-    (unspec_volatile:GPR
-      [(match_operand:GPR 1 "reg_or_0_operand" "rJ")
-       (match_operand:SI 2 "const_int_operand")]      ;; model
-      UNSPEC_ATOMIC_STORE))]
-  "TARGET_ATOMIC"
-  "amoswap.<amo>%A2 zero,%z1,%0"
-  [(set (attr "length") (const_int 8))])
-
 (define_insn "atomic_<atomic_optab><mode>"
   [(set (match_operand:GPR 0 "memory_operand" "+A")
 	(unspec_volatile:GPR