[PATCH] RISC-V: Fix feature_bits.c failed to compile on non-Linux targets

[Yangyu Chen]

The feature_bits.c file failed to compile on non-Linux targets because
we forgot to remove the __riscv_vendor_feature_bits.vendorID set
when target is not Linux.

This commit fixed this and also has several improvements including:
- Initialize all data to zero when syscall is not supported.
- Add detailed comments on processing implied extensions.

Fixes: ca44eb7f6a33 ("RISC-V: Implement __init_riscv_feature_bits,
                      __riscv_feature_bits, and
                      __riscv_vendor_feature_bits")

libgcc/ChangeLog:

        * config/riscv/feature_bits.c (__init_riscv_features_bits_linux):
          Remove __riscv_vendor_feature_bits.vendorID set when target is
          not Linux, and initialize all data to zero when syscall is not
          supported.

Signed-off-by: Yangyu Chen <chenyan...@isrc.iscas.ac.cn>
---
 libgcc/config/riscv/feature_bits.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/libgcc/config/riscv/feature_bits.c 
b/libgcc/config/riscv/feature_bits.c
index 9bdbc466fee..44f8eeee4e7 100644
--- a/libgcc/config/riscv/feature_bits.c
+++ b/libgcc/config/riscv/feature_bits.c
@@ -262,7 +262,15 @@ static void __init_riscv_features_bits_linux ()
                             0, 0);
 
   if (res)
-    return;
+    {
+      /* When syscall is not supported, just initialize that into all zeros.  
*/
+      __riscv_feature_bits.length = 0;
+      __riscv_vendor_feature_bits.length = 0;
+      __riscv_cpu_model.mvendorid = 0;
+      __riscv_cpu_model.marchid = 0;
+      __riscv_cpu_model.mimpid = 0;
+      return;
+    }
 
   const struct riscv_hwprobe hwprobe_mvendorid = hwprobes[0];
 
@@ -290,9 +298,12 @@ static void __init_riscv_features_bits_linux ()
     }
 
   const struct riscv_hwprobe hwprobe_ima_ext = hwprobes[4];
-
   /* Every time we add new extensions, we should check if previous extensions
-     imply the new extension and set the corresponding bit.  */
+     imply the new extension and set the corresponding bit.
+     We don't need to handle cases where:
+     1.  The new extension implies a previous extension (e.g., Zve32f -> F).
+     2.  The extensions imply some other extensions appear in the same release
+        version of Linux Kernel (e.g., Zbc - > Zbkc).  */
 
   if (hwprobe_ima_ext.value & RISCV_HWPROBE_IMA_FD)
     {
@@ -397,7 +408,7 @@ __init_riscv_feature_bits ()
 #ifdef __linux
   __init_riscv_features_bits_linux ();
 #else
-  /* Unsupported, just initlizaed that into all zeros.  */
+  /* Unsupported, just initialize that into all zeros.  */
   __riscv_feature_bits.length = 0;
   __riscv_vendor_feature_bits.length = 0;
   __riscv_cpu_model.mvendorid = 0;
-- 
2.45.2