[gcc(refs/vendors/riscv/heads/gcc-15-with-riscv-opts)] [RISC-V] Fix false positive from Wuninitialized

[Jeff Law]

https://gcc.gnu.org/g:57234351e1fec1164f68b5b63c281d6d90dc95b4

commit 57234351e1fec1164f68b5b63c281d6d90dc95b4
Author: Jeff Law <jlaw@ventanamicro.com>
Date:   Mon May 19 12:00:56 2025 -0600

    [RISC-V] Fix false positive from Wuninitialized
    
    As Mark and I independently tripped, there's a Wuninitialized issue in the
    RISC-V backend.  While *I* know the value would always be properly initialized,
    it'd be somewhat painful to either eliminate the infeasible paths or do deep
    enough analysis to suppress the false positive.
    
    So this initializes OUTPUT and verifies it's got a reasonable value before
    using it for the final copy into operands[0].
    
    Bootstrapped on the BPI (regression testing still has ~12hrs to go).
    
    gcc/
            * config/riscv/riscv.cc (synthesize_ior_xor): Initialize OUTPUT and
            verify it's non-null before emitting the final copy insn.
    
    (cherry picked from commit cbc258cd318756db8b5f0e4055dd8f1c1d618d22)

Diff:
---
 gcc/config/riscv/riscv.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index b511f70b6ada..c45c1683860a 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -14438,7 +14438,7 @@ synthesize_ior_xor (rtx_code code, rtx operands[3])
   /* Synthesis is better than loading the constant.  */
   ival = INTVAL (operands[2]);
   rtx input = operands[1];
-  rtx output;
+  rtx output = NULL_RTX;
 
   /* Emit the [x]ori insn that sets the low 11 bits into
      the proper state.  */
@@ -14467,6 +14467,8 @@ synthesize_ior_xor (rtx_code code, rtx operands[3])
       input = output;
       ival &= ~tmpval;
     }
+
+  gcc_assert (output);
   emit_move_insn (operands[0], output);
   return true;
 }