[Bug middle-end/113921] Output register of an "asm volatile goto" is incorrectly clobbered/discarded

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-02-14

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So, GCC 11 version:
--- gcc/cfgexpand.c.jj  2023-05-09 12:59:04.381738365 +0200
+++ gcc/cfgexpand.c     2024-02-14 19:56:08.733150432 +0100
@@ -3639,7 +3639,16 @@ expand_asm_stmt (gasm *stmt)
                emit_insn (copy_insn (PATTERN (curr)));
              rtx_insn *copy = get_insns ();
              end_sequence ();
-             insert_insn_on_edge (copy, e);
+             if (rtx_insn *prev = e->insns.r)
+               {
+                 /* Prepend copy before any other previously
+                    inserted insns on the edge rather than append.  */
+                 e->insns.r = NULL;
+                 insert_insn_on_edge (copy, e);
+                 insert_insn_on_edge (prev, e);
+               }
+             else
+               insert_insn_on_edge (copy, e);
            }
        }
     }
changes the emitted assembler:
@@ -7328,7 +7328,7 @@ sync_vmcs02_to_vmcs12:
 # 0 "" 2
 #NO_APP
 .L1127:
-       xorl    %r12d, %r12d
+       movq    %rax, %r12
 .L1083:
        movq    %r12, 240(%rbx)
        jmp     .L1047
@@ -29897,7 +29897,7 @@ nested_vmx_vmexit:
 # 0 "" 2
 #NO_APP
 .L5187:
-       xorl    %r12d, %r12d
+       movq    %rax, %r12
 .L5113:
        movq    %r12, %rdx
        movl    $7, %esi
which is I believe exactly what we want.

For GCC trunk the patch would be
--- gcc/cfgexpand.cc.jj 2024-02-10 11:25:09.995474027 +0100
+++ gcc/cfgexpand.cc    2024-02-14 19:54:30.811505882 +0100
@@ -3687,7 +3687,16 @@ expand_asm_stmt (gasm *stmt)
                  copy = get_insns ();
                  end_sequence ();
                }
-             insert_insn_on_edge (copy, e);
+             if (rtx_insn *prev = e->insns.r)
+               {
+                 /* Prepend copy before any other previously
+                    inserted insns on the edge rather than append.  */
+                 e->insns.r = NULL;
+                 insert_insn_on_edge (copy, e);
+                 insert_insn_on_edge (prev, e);
+               }
+             else
+               insert_insn_on_edge (copy, e);
            }
        }
     }
and with trunk it triggers (I mean the prev != NULL case) only on the
nested_vmx_vmexit
function and not the other one.
Guess one could try to build whole kernel with instrumented gcc (just add
FILE *f = fopen ("/tmp/asmgoto", "a");
fprintf (f, "%s %s\n", main_input_filename ? main_input_filename : "-",
current_function_name ());
fclose (f);
next to the e->insns.r = NULL; in the patch or so) to find out what else it
affects.