[PATCH] RISC-V: Fix use-after-free error in `parse_multiletter_ext'

[Maciej W. Rozycki]

Avoid undefined arithmetic involving a pointer to a heap allocation that 
has been freed and move a problematic calculation ahead of the following 
call to `free' in `riscv_subset_list::parse_multiletter_ext', removing a 
compilation error:

.../gcc/common/config/riscv/riscv-common.cc: In member function 'const char* 
riscv_subset_list::parse_multiletter_ext(const char*, const char*, const 
char*)':
.../gcc/common/config/riscv/riscv-common.cc:905:27: error: pointer 'subset' 
used after 'void free(void*)' [-Werror=use-after-free]
  905 |       p += end_of_version - subset;
      |            ~~~~~~~~~~~~~~~^~~~~~~~
.../gcc/common/config/riscv/riscv-common.cc:904:12: note: call to 'void 
free(void*)' here
  904 |       free (subset);
      |       ~~~~~^~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [Makefile:2428: riscv-common.o] Error 1

and a build regression from commit 671a283636de ("Add -Wuse-after-free 
[PR80532].").

        gcc/
        * common/config/riscv/riscv-common.cc 
        (riscv_subset_list::parse_multiletter_ext): Move pointer 
        arithmetic ahead of `free'.
---
 gcc/common/config/riscv/riscv-common.cc |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

gcc-riscv-parse-multiletter-ext-free.diff
Index: gcc/gcc/common/config/riscv/riscv-common.cc
===================================================================
--- gcc.orig/gcc/common/config/riscv/riscv-common.cc
+++ gcc/gcc/common/config/riscv/riscv-common.cc
@@ -901,8 +901,8 @@ riscv_subset_list::parse_multiletter_ext
        }
 
       add (subset, major_version, minor_version, explicit_version_p, false);
-      free (subset);
       p += end_of_version - subset;
+      free (subset);
 
       if (*p != '\0' && *p != '_')
        {