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Re: df_insn_refs_record's handling of global_regs[]


On 10/16/07, David Miller <davem@davemloft.net> wrote:
> From: "Seongbae Park (박성배, 朴成培)" <seongbae.park@gmail.com>
> Date: Tue, 16 Oct 2007 21:53:37 -0700
>
> Annyoung haseyo, Park-sanseng-nim,

:)

> > loop-invariant.cc uses ud-chain.
> > So if there's something wrong with the chain,
> > it could go nuts.
> > Can you send me the rtl dump of loop2_invariant pass ?
>
> I have found the problem, and yes it has to do with the ud chains.
>
> Because global registers are only marked via df_invalidated_by_call,
> they get the DF_REF_MAY_CLOBBER flag.
>
> This flag causes the dataflow problem solver to not add the global
> register definitions to the generator set.  Specifically I am
> talking about the code in df_rd_bb_local_compute_process_def(), it
> says:
>
>         if (!(DF_REF_FLAGS (def)
>               & (DF_REF_MUST_CLOBBER | DF_REF_MAY_CLOBBER)))
>           bitmap_set_bit (bb_info->gen, DF_REF_ID (def));
>
> Global registers don't get clobbered by calls, they are potentially
> set as a side effect of calling them.  And they are set to valid
> values we might actually depend upon as inputs later.
>
> I tried a potential fix, which is to change df_insn_refs_record(),
> such that it handles global registers instead like this:
>
>         if (global_regs[i])
>           df_ref_record (dflow, regno_reg_rtx[i], &regno_reg_rtx[i],
>                          bb, insn, DF_REF_REG_DEF, 0, true);
>
> and this made the illegal loop-invariant transformation no longer
> occur in my test case.

Did you replace the DF_REF_REG_USE with DEF ?
If so, that's not correct.  We need to add DEF as well as USE:

diff -r fd0f94fbe89d gcc/df-scan.c
--- a/gcc/df-scan.c     Wed Oct 10 03:32:43 2007 +0000
+++ b/gcc/df-scan.c     Tue Oct 16 22:52:44 2007 -0700
@@ -3109,8 +3109,13 @@ df_get_call_refs (struct df_collection_r
      so they are recorded as used.  */
   for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
     if (global_regs[i])
-      df_ref_record (collection_rec, regno_reg_rtx[i],
-                    NULL, bb, insn, DF_REF_REG_USE, flags);
+      {
+        df_ref_record (collection_rec, regno_reg_rtx[i],
+                      NULL, bb, insn, DF_REF_REG_USE, flags);
+        df_ref_record (collection_rec, regno_reg_rtx[i],
+                      NULL, bb, insn, DF_REF_REG_DEF, flags);
+      }
+

   is_sibling_call = SIBLING_CALL_P (insn);
   EXECUTE_IF_SET_IN_BITMAP (df_invalidated_by_call, 0, ui, bi)


Then, we'll need to change the df_invalidated_by_call loop
not to add global_regs[] again (with MAY_CLOBBER bits).
-- 
#pragma ident "Seongbae Park, compiler, http://seongbae.blogspot.com";

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