Re: GCC 4.2.1 produces broken 2.6.22.1 kernel.

[David Miller]

Better late than never...

Ok, I think I nailed this, the simplest test case is:

register unsigned long regval asm("g5");

extern void cond_resched(void);

unsigned int var;

void *expression(unsigned long regval)
{
  void *ret;

  __asm__("" : "=r" (ret) : "0" (&var));
  return ret + regval;
}

void func(void **pp)
{
  int i;

  for (i = 0; i < 56; i++) {
    cond_resched();
    *pp = expression(regval);
  }
}

If you build that with "gcc-4.2 -m64 -Os -S -o test.s test.c"
the compiler moves expression() outside of the loop because it
things it is invariant.

The dataflow pass doesn't mark global registers correctly, and
the new thing in gcc-4.2.x vs. gcc-4.1.x is that it uses the
dataflow pass for the loop optimizers.

I've bootstrapped and regression tested the following gcc-4.2.x
patch, I'll test some test kernel builds after I get some sleep.

--- ./gcc/df-scan.c.ORIG        2007-10-16 02:07:46.000000000 -0700
+++ ./gcc/df-scan.c     2007-10-16 23:00:32.000000000 -0700
@@ -1584,12 +1584,19 @@ df_insn_refs_record (struct dataflow *df
                 so they are recorded as used.  */
              for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
                if (global_regs[i])
-                 df_uses_record (dflow, &regno_reg_rtx[i],
-                                 DF_REF_REG_USE, bb, insn, 
-                                 0);
+                 {
+                   df_uses_record (dflow, &regno_reg_rtx[i],
+                                   DF_REF_REG_USE, bb, insn, 0);
+                   df_ref_record (dflow, regno_reg_rtx[i], &regno_reg_rtx[i],
+                                  bb, insn, DF_REF_REG_DEF, 0, true);
+                 }
+
              EXECUTE_IF_SET_IN_BITMAP (df_invalidated_by_call, 0, ui, bi)
-               df_ref_record (dflow, regno_reg_rtx[ui], &regno_reg_rtx[ui], 
bb, 
-                              insn, DF_REF_REG_DEF, DF_REF_MAY_CLOBBER, false);
+               {
+                 if (!global_regs[ui])
+                   df_ref_record (dflow, regno_reg_rtx[ui], 
&regno_reg_rtx[ui], bb, 
+                                  insn, DF_REF_REG_DEF, DF_REF_MAY_CLOBBER, 
false);
+               }
            }
        }
 
-
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