[gcc r16-1541] [PATCH v1] RISC-V: Use scratch reg for loop control

[Jeff Law]

https://gcc.gnu.org/g:a22ed5658797cc9ca4421e69db5d6259389a4156

commit r16-1541-ga22ed5658797cc9ca4421e69db5d6259389a4156
Author: Umesh Kalappa <ukalappa.mips@gmail.com>
Date:   Tue Jun 17 07:23:41 2025 -0600

    [PATCH v1] RISC-V: Use scratch reg for loop control
    
    By using the scratch register for loop control rather than the output
    of the lr instruction we can avoid an unnecessary "mv" instruction.
    
    --
    V2: Testcase update with no regressions found for the following the changes.
    
    gcc/ChangeLog:
    
            * config/riscv/sync.md (lrsc_atomic_exchange<mode>): Use scratch
            register for loop control rather than lr output.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.target/riscv/zalrsc.c: New test.

Diff:
---
 gcc/config/riscv/sync.md                | 11 +++++------
 gcc/testsuite/gcc.target/riscv/zalrsc.c | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/gcc/config/riscv/sync.md b/gcc/config/riscv/sync.md
index 726800a96623..a75ea6834e46 100644
--- a/gcc/config/riscv/sync.md
+++ b/gcc/config/riscv/sync.md
@@ -405,18 +405,17 @@
 	   (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"))
    (clobber (match_scratch:GPR 4 "=&r"))]	  ;; tmp_1
   "!TARGET_ZAAMO && TARGET_ZALRSC"
   {
     return "1:\;"
-	   "lr.<amo>%I3\t%4, %1\;"
-	   "sc.<amo>%J3\t%0, %0, %1\;"
-	   "bnez\t%0, 1b\;"
-	   "mv\t%0, %4";
+	   "lr.<amo>%I3\t%0, %1\;"
+	   "sc.<amo>%J3\t%4, %z2, %1\;"
+	   "bnez\t%4, 1b\";
   }
   [(set_attr "type" "atomic")
-   (set (attr "length") (const_int 16))])
+   (set (attr "length") (const_int 12))])
 
 (define_expand "atomic_exchange<mode>"
   [(match_operand:SHORT 0 "register_operand") ;; old value at mem
diff --git a/gcc/testsuite/gcc.target/riscv/zalrsc.c b/gcc/testsuite/gcc.target/riscv/zalrsc.c
new file mode 100644
index 000000000000..19a26bfb47ce
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zalrsc.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64imfd_zalrsc -mabi=lp64" } */
+/* { dg-skip-if "" { *-*-* } {"-O0"} } */
+
+/* lr.w/sc.w */
+int *i;
+int lr_sc(int v)
+{
+  return __atomic_exchange_4(i, v, __ATOMIC_RELAXED);
+}
+
+/* { dg-final { scan-assembler-times {\mlr.w} 1 } } */
+/* { dg-final { scan-assembler-times {\msc.w} 1 } } */
+/* { dg-final { scan-assembler-not   {"mv\t"}   } } */