[gcc(refs/vendors/riscv/heads/gcc-15-with-riscv-opts)] [PR target/121213] Avoid unnecessary constant load in amoswap

[Jeff Law]

https://gcc.gnu.org/g:ad1addbbef23629d500372a56ad04b1d31f148fb

commit ad1addbbef23629d500372a56ad04b1d31f148fb
Author: Austin Law <austinklaw@gmail.com>
Date:   Sun Aug 17 09:03:51 2025 -0600

    [PR target/121213] Avoid unnecessary constant load in amoswap
    
    PR 121213 shows an unnecessary "li target,0" in an atomic exchange loop
    on RISC-V.
    
    The source operand for an amoswap instruction should allow (const_int 0)
    in addition to GPRs.  So the operand's predicate is changed to
    "reg_or_0_operand".   The corresponding constraint is also changed to
    allow a reg or the constant 0.
    
    With the source operand no longer tied to the destination operand we do
    not need the earlyclobber for the destination, so the destination
    operand's constraint is adjusted accordingly.
    
    This patch does not address the unnecessary sign extension reported in
    the PR.
    
    Tested with no regressions on riscv32-elf and riscv64-elf.
    
            PR target/121213
    gcc/
            * config/riscv/sync.md (amo_atomic_exchange<mode>): Allow
            (const_int 0) as input operand. Do not tie input to output.
            No longer earlyclobber the output.
    
    gcc/testsuite
            * gcc.target/riscv/amo/pr121213.c: New test.
    
    (cherry picked from commit 6f63044a7ae63a276a4f6d3108849e093c690bc6)

Diff:
---
 gcc/config/riscv/sync.md                      |  8 ++++----
 gcc/testsuite/gcc.target/riscv/amo/pr121213.c | 17 +++++++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/gcc/config/riscv/sync.md b/gcc/config/riscv/sync.md
index 50ec8b38f723..e47bb41adcc2 100644
--- a/gcc/config/riscv/sync.md
+++ b/gcc/config/riscv/sync.md
@@ -386,13 +386,13 @@
   })
 
 (define_insn "amo_atomic_exchange<mode>"
-  [(set (match_operand:GPR 0 "register_operand" "=&r")
+  [(set (match_operand:GPR 0 "register_operand" "=r")
 	(unspec_volatile:GPR
 	  [(match_operand:GPR 1 "memory_operand" "+A")
 	   (match_operand:SI 3 "const_int_operand")] ;; model
 	  UNSPEC_SYNC_EXCHANGE))
    (set (match_dup 1)
-	(match_operand:GPR 2 "register_operand" "0"))]
+	(match_operand:GPR 2 "reg_or_0_operand" "rJ"))]
   "TARGET_ZAAMO"
   "amoswap.<amo>%A3\t%0,%z2,%1"
   [(set_attr "type" "atomic")
@@ -434,13 +434,13 @@
 })
 
 (define_insn "zabha_atomic_exchange<mode>"
-  [(set (match_operand:SHORT 0 "register_operand" "=&r")
+  [(set (match_operand:SHORT 0 "register_operand" "=r")
 	(unspec_volatile:SHORT
 	  [(match_operand:SHORT 1 "memory_operand" "+A")
 	   (match_operand:SI 3 "const_int_operand")] ;; model
 	  UNSPEC_SYNC_EXCHANGE_ZABHA))
    (set (match_dup 1)
-	(match_operand:SHORT 2 "register_operand" "0"))]
+	(match_operand:SHORT 2 "reg_or_0_operand" "rJ"))]
   "TARGET_ZABHA"
   "amoswap.<amobh>%A3\t%0,%z2,%1"
   [(set_attr "type" "atomic")
diff --git a/gcc/testsuite/gcc.target/riscv/amo/pr121213.c b/gcc/testsuite/gcc.target/riscv/amo/pr121213.c
new file mode 100644
index 000000000000..3b2d694f9914
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/amo/pr121213.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=rv64gc -mabi=lp64" { target { rv64 } } } */
+/* { dg-options "-O2 -march=rv32gc -mabi=ilp32" { target { rv32 } } } */
+
+void test0(unsigned long* lock) {
+  while (!__atomic_exchange_n(lock, 0, __ATOMIC_ACQUIRE));
+}
+
+
+void test1(unsigned* lock) {
+  while (!__atomic_exchange_n(lock, 0, __ATOMIC_ACQUIRE));
+}
+
+/* { dg-final { scan-assembler-not "\tli" } } */
+/* { dg-final { scan-assembler-times "\tamoswap...aq\t\[axt\]\[0-9\],zero," 2 } } */
+/* { dg-final { scan-assembler-not "\tsext" { xfail *-*-* } } } */
+