lra: set insn_code_data to NULL when freeing

libgccjit's test-threads.c repeatedly compiles and runs numerous tests,
each in a separate thread.

Attempting to add an empty test that generates no code leads to a
double-free ICE within that thread, within lra.c's
finish_insn_code_data_once.

The root cause is that the insn_code_data array is cleared in
init_insn_code_data_once, but this is only called the first time
a cgraph_node is expanded [1], whereas the "loop-over-all-elements
and free them" is unconditionally called in finalize [2].  Hence
if there are no functions:
* the array is not re-initialized for the empty context
* when finish_insn_code_data_once is called for the empty context
it still contains the freed pointers from the previous context
that held the jit mutex, and hence the free is a double-free.

This patch sets the pointers to NULL after freeing them, fixing
the ICE.

[1]
init_insn_code_data_once is called via
  lra_init_once called by
    ira_init_once called by
      initialize_rtl, via:
         if (!rtl_initialized)
           ira_init_once ();
        called by init_function_start
           called by cgraph_node::expand

[2]:
finish_insn_code_data_once is called by:
  lra_finish_once called by
    finalize

gcc/ChangeLog:
	* lra.c (finish_insn_code_data_once): Set the array elements
	to NULL after freeing them.

gcc/testsuite/ChangeLog:
	* jit.dg/all-non-failing-tests.h: Add test-empty.c

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-30  David Malcolm  <dmalcolm@redhat.com>
+
+	* lra.c (finish_insn_code_data_once): Set the array elements
+	to NULL after freeing them.
+
 2020-03-30  Andreas Schwab  <schwab@suse.de>
 
 	* config/host-linux.c (TRY_EMPTY_VM_SPACE) [__riscv && __LP64__]:

gcc/lra.c

@@ -653,7 +653,10 @@ finish_insn_code_data_once (void)
   for (unsigned int i = 0; i < NUM_INSN_CODES; i++)
     {
       if (insn_code_data[i] != NULL)
-	free (insn_code_data[i]);
+	{
+	  free (insn_code_data[i]);
+	  insn_code_data[i] = NULL;
+	}
     }
 }
 

gcc/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-30  David Malcolm  <dmalcolm@redhat.com>
+
+	* jit.dg/all-non-failing-tests.h: Add test-empty.c
+
 2020-03-30  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/94385

gcc/testsuite/jit.dg/all-non-failing-tests.h

@@ -116,6 +116,13 @@
 #undef create_code
 #undef verify_code
 
+/* test-empty.c */
+#define create_code create_code_empty
+#define verify_code verify_code_empty
+#include "test-empty.c"
+#undef create_code
+#undef verify_code
+
 /* test-error-*.c: We don't use these test cases, since they deliberately
    introduce errors, which we don't want here.  */
 
@@ -328,6 +335,9 @@ const struct testcase testcases[] = {
   {"expressions",
    create_code_expressions,
    verify_code_expressions},
+  {"empty",
+   create_code_empty,
+   verify_code_empty},
   {"factorial",
    create_code_factorial,
    verify_code_factorial},