[PATCH] libgcc: Implement -mx32 [PR113855]

[Jakub Jelinek]

On Sat, Feb 10, 2024 at 10:05:34AM -0800, H.J. Lu wrote:
> > I bet it probably doesn't work properly for -mx32 (which defines
> > __x86_64__), CCing H.J. on that, but that is a preexisting issue
> > (and I don't have any experience with it; I guess one would either
> > need to add 4 bytes of padding after the func_ptr so that those
> > bits remain zeros as sizeof (void *) is 4, but presumably it would be
> > better to just use movl (but into %r10) and maybe the jmpl instead
> > of movabsq.
> 
> Are there any testcases to exercise this code on Linux?

Here is an untested attempt to implement it for -mx32 (well, I've compiled
it with -mx32 in libgcc by hand after stubbing
/usr/include/gnu/stubs-x32.h).

Testcase could be something like:

/* { dg-do run } */
/* { dg-options "-ftrampoline-impl=heap" } */

__attribute__((noipa)) int
bar (int (*fn) (int))
{
  return fn (42) + 1;
}

int
main ()
{
  int a = 0;
  int foo (int x) { if (x != 42) __builtin_abort (); return ++a; }
  if (bar (foo) != 2 || a != 1)
    __builtin_abort ();
  if (bar (foo) != 3 || a != 2)
    __builtin_abort ();
  a = 42;
  if (bar (foo) != 44 || a != 43)
    __builtin_abort ();
  return 0;
}
but I must say I'm also surprised we have no tests for this in the
testsuite.  Sure, we'd also need to add some effective target whether
-ftrampoline-impl=heap can be used for a link/runtime test or not.

2024-02-13  Jakub Jelinek  <ja...@redhat.com>

        PR target/113855
        * config/i386/heap-trampoline.c (trampoline_insns): Use movabsq
        instead of movabs in comments.  Add -mx32 variant.

--- libgcc/config/i386/heap-trampoline.c.jj     2024-02-12 18:48:08.548886036 
+0100
+++ libgcc/config/i386/heap-trampoline.c        2024-02-13 10:24:06.348020670 
+0100
@@ -30,18 +30,34 @@ void __gcc_nested_func_ptr_created (void
 void __gcc_nested_func_ptr_deleted (void);
 
 #if __x86_64__
+
+#ifdef __LP64__
 static const uint8_t trampoline_insns[] = {
-  /* movabs $<func>,%r11  */
+  /* movabsq $<func>,%r11  */
   0x49, 0xbb,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 
-  /* movabs $<chain>,%r10  */
+  /* movabsq $<chain>,%r10  */
   0x49, 0xba,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 
   /* rex.WB jmpq *%r11  */
   0x41, 0xff, 0xe3
 };
+#else
+static const uint8_t trampoline_insns[] = {
+  /* movl $<func>,%r11d  */
+  0x41, 0xbb,
+  0x00, 0x00, 0x00, 0x00,
+
+  /* movl $<chain>,%r10d  */
+  0x41, 0xba,
+  0x00, 0x00, 0x00, 0x00,
+
+  /* rex.WB jmpq *%r11  */
+  0x41, 0xff, 0xe3
+};
+#endif
 
 union ix86_trampoline {
   uint8_t insns[sizeof(trampoline_insns)];


        Jakub