[gcc r17-3005] RISC-V: Return implied_exts by const reference

https://gcc.gnu.org/g:487ed3d78a7725304da0d67b557c5dee093bf384

commit r17-3005-g487ed3d78a7725304da0d67b557c5dee093bf384
Author: Jin Ma <[email protected]>
Date:   Wed Jul 22 15:49:51 2026 +0800

    RISC-V: Return implied_exts by const reference
    
    riscv_ext_info_t::implied_exts() returns its std::vector member by
    value, but all three callers (check_implied_ext, handle_combine_ext,
    riscv_minimal_hwprobe_feature_bits) only iterate over the result and
    never need ownership.  Every invocation therefore copies the entire
    implied-extension vector.  During option parsing this runs once per
    compiled source file; at -O0 the optimisation passes are skipped so
    option-parsing overhead dominates, causing a 6.7% compile-time
    regression on CSIBE -O0 benchmarks.
    
    Return a const reference instead.  The underlying m_implied_exts
    member lives in a static global map entry and outlives every caller,
    so no dangling-reference risk exists.
    
    gcc/ChangeLog:
    
            * common/config/riscv/riscv-common.cc
            (riscv_ext_info_t::implied_exts): Return const reference
            instead of by-value copy.
    
    Signed-off-by: Jin Ma <[email protected]>

Diff:
---
 gcc/common/config/riscv/riscv-common.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/common/config/riscv/riscv-common.cc b/gcc/common/config/riscv/riscv-common.cc
index 2d56ba59f0e4..6bbbacab8df4 100644
--- a/gcc/common/config/riscv/riscv-common.cc
+++ b/gcc/common/config/riscv/riscv-common.cc
@@ -135,7 +135,7 @@ public:
   /* Return true if any change.  */
   bool apply_implied_ext (riscv_subset_list *subset_list) const;
 
-  const std::vector<riscv_implied_info_t> implied_exts () const
+  const std::vector<riscv_implied_info_t> &implied_exts () const
   {
     return m_implied_exts;
   }