Fix reload1.c warning for some targets

Fix reload1.c warning for some targets

[Richard Sandiford]

Building some targets results in a warning about orig_dup[i] potentially
being used uninitialised.  I think the warning is fair, since it isn't
obvious that the reog_data-based loop bound remains unchanged between:

  for (i = 0; i < recog_data.n_dups; i++)
    orig_dup[i] = *recog_data.dup_loc[i];

and:

  for (i = 0; i < recog_data.n_dups; i++)
    *recog_data.dup_loc[i] = orig_dup[i];

Tested on x86_64-linux-gnu.  OK to install?

Thanks,
Richard

gcc/
	* reload1.c (elimination_costs_in_insn): Make it obvious to the
	compiler that the n_dups and n_operands loop bounds are invariant.

diff --git a/gcc/reload1.c b/gcc/reload1.c
index ce06e06..ad243e3 100644
--- a/gcc/reload1.c
+++ b/gcc/reload1.c
@@ -3708,10 +3708,12 @@ elimination_costs_in_insn (rtx_insn *insn)
   /* Eliminate all eliminable registers occurring in operands that
      can be handled by reload.  */
   extract_insn (insn);
-  for (i = 0; i < recog_data.n_dups; i++)
+  int n_dups = recog_data.n_dups;
+  for (i = 0; i < n_dups; i++)
     orig_dup[i] = *recog_data.dup_loc[i];
 
-  for (i = 0; i < recog_data.n_operands; i++)
+  int n_operands = recog_data.n_operands;
+  for (i = 0; i < n_operands; i++)
     {
       orig_operand[i] = recog_data.operand[i];
 
@@ -3756,7 +3758,7 @@ elimination_costs_in_insn (rtx_insn *insn)
 	}
     }
 
-  for (i = 0; i < recog_data.n_dups; i++)
+  for (i = 0; i < n_dups; i++)
     *recog_data.dup_loc[i]
       = *recog_data.operand_loc[(int) recog_data.dup_num[i]];
 
@@ -3764,9 +3766,9 @@ elimination_costs_in_insn (rtx_insn *insn)
   check_eliminable_occurrences (old_body);
 
   /* Restore the old body.  */
-  for (i = 0; i < recog_data.n_operands; i++)
+  for (i = 0; i < n_operands; i++)
     *recog_data.operand_loc[i] = orig_operand[i];
-  for (i = 0; i < recog_data.n_dups; i++)
+  for (i = 0; i < n_dups; i++)
     *recog_data.dup_loc[i] = orig_dup[i];
 
   /* Update all elimination pairs to reflect the status after the current

Re: Fix reload1.c warning for some targets

[Jeff Law]

On 08/05/2015 08:18 AM, Richard Sandiford wrote:
> Building some targets results in a warning about orig_dup[i] potentially
> being used uninitialised.  I think the warning is fair, since it isn't
> obvious that the reog_data-based loop bound remains unchanged between:
>
>    for (i = 0; i < recog_data.n_dups; i++)
>      orig_dup[i] = *recog_data.dup_loc[i];
>
> and:
>
>    for (i = 0; i < recog_data.n_dups; i++)
>      *recog_data.dup_loc[i] = orig_dup[i];
>
> Tested on x86_64-linux-gnu.  OK to install?
>
> Thanks,
> Richard
>
> gcc/
> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
> 	compiler that the n_dups and n_operands loop bounds are invariant.
There's a BZ about this issue.  55035.

What I want is to make sure we don't lose track of the false positive in 
55035 (caused by a miss jump thread due to aliasing issues).

So perhaps the way forward is to install your change and twiddle the 
summary of 55035 in some way that makes it more obvious the bz tracks a 
false positive from -Wuninitialized and attach 55035 to the 
-Wuninitialized meta bug (24639).

Does that work for you?

Jeff

Re: Fix reload1.c warning for some targets

[Richard Sandiford]

Jeff Law <law@redhat.com> writes:
> On 08/05/2015 08:18 AM, Richard Sandiford wrote:
>> Building some targets results in a warning about orig_dup[i] potentially
>> being used uninitialised.  I think the warning is fair, since it isn't
>> obvious that the reog_data-based loop bound remains unchanged between:
>>
>>    for (i = 0; i < recog_data.n_dups; i++)
>>      orig_dup[i] = *recog_data.dup_loc[i];
>>
>> and:
>>
>>    for (i = 0; i < recog_data.n_dups; i++)
>>      *recog_data.dup_loc[i] = orig_dup[i];
>>
>> Tested on x86_64-linux-gnu.  OK to install?
>>
>> Thanks,
>> Richard
>>
>> gcc/
>> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
>> 	compiler that the n_dups and n_operands loop bounds are invariant.
> There's a BZ about this issue.  55035.
>
> What I want is to make sure we don't lose track of the false positive in 
> 55035 (caused by a miss jump thread due to aliasing issues).
>
> So perhaps the way forward is to install your change and twiddle the 
> summary of 55035 in some way that makes it more obvious the bz tracks a 
> false positive from -Wuninitialized and attach 55035 to the 
> -Wuninitialized meta bug (24639).

Is it really a false positive in this case though?  We have:

  for (i = 0; i < recog_data.n_dups; i++)
    orig_dup[i] = *recog_data.dup_loc[i];

  for (i = 0; i < recog_data.n_operands; i++)
    {
      orig_operand[i] = recog_data.operand[i];

      /* For an asm statement, every operand is eliminable.  */
      if (insn_is_asm || insn_data[icode].operand[i].eliminable)
	{
	  bool is_set_src, in_plus;

	  /* Check for setting a register that we know about.  */
	  if (recog_data.operand_type[i] != OP_IN
	      && REG_P (orig_operand[i]))
	    {
	      /* If we are assigning to a register that can be eliminated, it
		 must be as part of a PARALLEL, since the code above handles
		 single SETs.  We must indicate that we can no longer
		 eliminate this reg.  */
	      for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS];
		   ep++)
		if (ep->from_rtx == orig_operand[i])
		  ep->can_eliminate = 0;
	    }

	  /* Companion to the above plus substitution, we can allow
	     invariants as the source of a plain move.  */
	  is_set_src = false;
	  if (old_set && recog_data.operand_loc[i] == &SET_SRC (old_set))
	    is_set_src = true;
	  if (is_set_src && !sets_reg_p)
	    note_reg_elim_costly (SET_SRC (old_set), insn);
	  in_plus = false;
	  if (plus_src && sets_reg_p
	      && (recog_data.operand_loc[i] == &XEXP (plus_src, 0)
		  || recog_data.operand_loc[i] == &XEXP (plus_src, 1)))
	    in_plus = true;

	  eliminate_regs_1 (recog_data.operand[i], VOIDmode,
			    NULL_RTX,
			    is_set_src || in_plus, true);
	  /* Terminate the search in check_eliminable_occurrences at
	     this point.  */
	  *recog_data.operand_loc[i] = 0;
	}
    }

  for (i = 0; i < recog_data.n_dups; i++)
    *recog_data.dup_loc[i]
      = *recog_data.operand_loc[(int) recog_data.dup_num[i]];

  /* If any eliminable remain, they aren't eliminable anymore.  */
  check_eliminable_occurrences (old_body);

  /* Restore the old body.  */
  for (i = 0; i < recog_data.n_operands; i++)
    *recog_data.operand_loc[i] = orig_operand[i];
  for (i = 0; i < recog_data.n_dups; i++)
    *recog_data.dup_loc[i] = orig_dup[i];

and I don't see how GCC could prove that eliminate_regs_1 doesn't
modify the value of recog_data.n_dups between the two loops.
eliminate_regs_1 calls functions like plus_constant that are defined
outside the TU and that certainly aren't pure/const.

So I think c#5 (marked as a bogus reduction) is an accurate reflection
of what reload1.c does.  c#4 looks like a genuine bug but seems different
from the reload1.c case.  If we still warn for c#4 then I think we
should keep the bugzilla entry open for that, but the warning for the
reload1.c code seems justified.  Perhaps the question is why it doesn't
trigger on more targets :-)

Thanks,
Richard

Re: Fix reload1.c warning for some targets

[Jeff Law]

On 08/05/2015 11:32 AM, Richard Sandiford wrote:
> and I don't see how GCC could prove that eliminate_regs_1 doesn't
> modify the value of recog_data.n_dups between the two loops.
> eliminate_regs_1 calls functions like plus_constant that are defined
> outside the TU and that certainly aren't pure/const.
Right.  I should have been clearer.  I don't think the reload1.c code is 
a false positive because we can't see into those functions to determine 
side effects.

> So I think c#5 (marked as a bogus reduction) is an accurate reflection
> of what reload1.c does.  c#4 looks like a genuine bug but seems different
> from the reload1.c case.  If we still warn for c#4 then I think we
> should keep the bugzilla entry open for that, but the warning for the
> reload1.c code seems justified.
Right.  I don't want to lose the false positive and associated missed 
jump threading in c#4.


  Perhaps the question is why it doesn't trigger on more targets :-)
Not sure.  Could be how match_dup is used plus some interactions with 
SRA and BRANCH_COST and who knows what else :-0


Jeff

Re: Fix reload1.c warning for some targets

[Jeff Law]

On 08/05/2015 08:18 AM, Richard Sandiford wrote:
> Building some targets results in a warning about orig_dup[i] potentially
> being used uninitialised.  I think the warning is fair, since it isn't
> obvious that the reog_data-based loop bound remains unchanged between:
>
>    for (i = 0; i < recog_data.n_dups; i++)
>      orig_dup[i] = *recog_data.dup_loc[i];
>
> and:
>
>    for (i = 0; i < recog_data.n_dups; i++)
>      *recog_data.dup_loc[i] = orig_dup[i];
>
> Tested on x86_64-linux-gnu.  OK to install?
>
> Thanks,
> Richard
>
> gcc/
> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
> 	compiler that the n_dups and n_operands loop bounds are invariant.
So thinking more about this, I think the best way forward is to:

   1. Create a new BZ with the false positive extracted from c#4.

   2. Install your patch and close 55035.

I'll take care of #1, you can handle #2.


jeff

Re: Fix reload1.c warning for some targets

[Richard Sandiford]

Jeff Law <law@redhat.com> writes:
> On 08/05/2015 08:18 AM, Richard Sandiford wrote:
>> Building some targets results in a warning about orig_dup[i] potentially
>> being used uninitialised.  I think the warning is fair, since it isn't
>> obvious that the reog_data-based loop bound remains unchanged between:
>>
>>    for (i = 0; i < recog_data.n_dups; i++)
>>      orig_dup[i] = *recog_data.dup_loc[i];
>>
>> and:
>>
>>    for (i = 0; i < recog_data.n_dups; i++)
>>      *recog_data.dup_loc[i] = orig_dup[i];
>>
>> Tested on x86_64-linux-gnu.  OK to install?
>>
>> Thanks,
>> Richard
>>
>> gcc/
>> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
>> 	compiler that the n_dups and n_operands loop bounds are invariant.
> So thinking more about this, I think the best way forward is to:
>
>    1. Create a new BZ with the false positive extracted from c#4.
>
>    2. Install your patch and close 55035.
>
> I'll take care of #1, you can handle #2.

Thanks, I've now done #2.

Richard

Re: Fix reload1.c warning for some targets

[Jeff Law]

On 08/13/2015 02:29 PM, Richard Sandiford wrote:
> Jeff Law <law@redhat.com> writes:
>> On 08/05/2015 08:18 AM, Richard Sandiford wrote:
>>> Building some targets results in a warning about orig_dup[i] potentially
>>> being used uninitialised.  I think the warning is fair, since it isn't
>>> obvious that the reog_data-based loop bound remains unchanged between:
>>>
>>>     for (i = 0; i < recog_data.n_dups; i++)
>>>       orig_dup[i] = *recog_data.dup_loc[i];
>>>
>>> and:
>>>
>>>     for (i = 0; i < recog_data.n_dups; i++)
>>>       *recog_data.dup_loc[i] = orig_dup[i];
>>>
>>> Tested on x86_64-linux-gnu.  OK to install?
>>>
>>> Thanks,
>>> Richard
>>>
>>> gcc/
>>> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
>>> 	compiler that the n_dups and n_operands loop bounds are invariant.
>> So thinking more about this, I think the best way forward is to:
>>
>>     1. Create a new BZ with the false positive extracted from c#4.
>>
>>     2. Install your patch and close 55035.
>>
>> I'll take care of #1, you can handle #2.
>
> Thanks, I've now done #2.
THanks.  I've got the new BZ in place.  So we both pop one item off our 
stacks.

jeff

Re: Fix reload1.c warning for some targets

[Rainer Orth]

Richard Sandiford <rdsandiford@googlemail.com> writes:

> Jeff Law <law@redhat.com> writes:
>> On 08/05/2015 08:18 AM, Richard Sandiford wrote:
>>> Building some targets results in a warning about orig_dup[i] potentially
>>> being used uninitialised.  I think the warning is fair, since it isn't
>>> obvious that the reog_data-based loop bound remains unchanged between:
>>>
>>>    for (i = 0; i < recog_data.n_dups; i++)
>>>      orig_dup[i] = *recog_data.dup_loc[i];
>>>
>>> and:
>>>
>>>    for (i = 0; i < recog_data.n_dups; i++)
>>>      *recog_data.dup_loc[i] = orig_dup[i];
>>>
>>> Tested on x86_64-linux-gnu.  OK to install?
>>>
>>> Thanks,
>>> Richard
>>>
>>> gcc/
>>> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
>>> 	compiler that the n_dups and n_operands loop bounds are invariant.
>> So thinking more about this, I think the best way forward is to:
>>
>>    1. Create a new BZ with the false positive extracted from c#4.
>>
>>    2. Install your patch and close 55035.
>>
>> I'll take care of #1, you can handle #2.
>
> Thanks, I've now done #2.

Unfortunately the patch broke sparcv9-sun-solaris2* (only,
sparc-sun-solaris2* is fine) bootstrap:

/vol/gcc/src/hg/trunk/local/gcc/reload1.c: In function 'void elimination_costs_in_insn(rtx_insn*)':
/vol/gcc/src/hg/trunk/local/gcc/reload1.c:3772:41: error: 'orig_dup[1]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     *recog_data.dup_loc[i] = orig_dup[i];
                                         ^
/vol/gcc/src/hg/trunk/local/gcc/reload1.c:3772:41: error: 'orig_dup[0]' may be used uninitialized in this function [-Werror=maybe-uninitialized]

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University

Re: Fix reload1.c warning for some targets

[Richard Sandiford]

Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE> writes:
> Richard Sandiford <rdsandiford@googlemail.com> writes:
>> Jeff Law <law@redhat.com> writes:
>>> On 08/05/2015 08:18 AM, Richard Sandiford wrote:
>>>> Building some targets results in a warning about orig_dup[i] potentially
>>>> being used uninitialised.  I think the warning is fair, since it isn't
>>>> obvious that the reog_data-based loop bound remains unchanged between:
>>>>
>>>>    for (i = 0; i < recog_data.n_dups; i++)
>>>>      orig_dup[i] = *recog_data.dup_loc[i];
>>>>
>>>> and:
>>>>
>>>>    for (i = 0; i < recog_data.n_dups; i++)
>>>>      *recog_data.dup_loc[i] = orig_dup[i];
>>>>
>>>> Tested on x86_64-linux-gnu.  OK to install?
>>>>
>>>> Thanks,
>>>> Richard
>>>>
>>>> gcc/
>>>> 	* reload1.c (elimination_costs_in_insn): Make it obvious to the
>>>> 	compiler that the n_dups and n_operands loop bounds are invariant.
>>> So thinking more about this, I think the best way forward is to:
>>>
>>>    1. Create a new BZ with the false positive extracted from c#4.
>>>
>>>    2. Install your patch and close 55035.
>>>
>>> I'll take care of #1, you can handle #2.
>>
>> Thanks, I've now done #2.
>
> Unfortunately the patch broke sparcv9-sun-solaris2* (only,
> sparc-sun-solaris2* is fine) bootstrap:
>
> /vol/gcc/src/hg/trunk/local/gcc/reload1.c: In function 'void elimination_costs_in_insn(rtx_insn*)':
> /vol/gcc/src/hg/trunk/local/gcc/reload1.c:3772:41: error: 'orig_dup[1]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      *recog_data.dup_loc[i] = orig_dup[i];
>                                          ^
> /vol/gcc/src/hg/trunk/local/gcc/reload1.c:3772:41: error: 'orig_dup[0]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>
> 	Rainer

Sorry for the slow reply.  I did try to coerce the current uninit
pass to handle this case, but the patch I ended up with only worked
by accident because of the strange way in which the pass handles
limit cases.  (If we have more than MAX_NUM_CHAINS chains, it silently
drops the excess chains and continues regardless, so it's quite easy
to come up with cases where the predicates for either the definition
or the use consider an arbitrary subset of the actual conditions.)

It sounds like Jeff has a much more radical rewrite in mind,
so for now how about just turning -Wmaybe-uninitialized into
a warning for this function?  The patch will mean that it becomes
a warning even if someone turns off warnings on the command line,
but I don't think that's important.

Bootstrapped and regression-tested on x86_64-linux-gnu.  Also tested
with a cross-compiler to sparc-linux-gnu (which also triggered the
warning for me).  Tested that clang could still compile the file.
OK to install?


gcc/
	* reload1.c (elimination_costs_in_insn): Locally turn
	-Wmaybe-uninitialized into a warning.

diff --git a/gcc/reload1.c b/gcc/reload1.c
index ad243e3..c7cc37b 100644
--- a/gcc/reload1.c
+++ b/gcc/reload1.c
@@ -3636,6 +3636,8 @@ eliminate_regs_in_insn (rtx_insn *insn, int replace)
    eliminations in its operands and record cases where eliminating a reg with
    an invariant equivalence would add extra cost.  */
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic warning "-Wmaybe-uninitialized"
 static void
 elimination_costs_in_insn (rtx_insn *insn)
 {
@@ -3785,6 +3787,7 @@ elimination_costs_in_insn (rtx_insn *insn)
 
   return;
 }
+#pragma GCC diagnostic pop
 
 /* Loop through all elimination pairs.
    Recalculate the number not at initial offset.

Re: Fix reload1.c warning for some targets

[Jeff Law]

On 09/03/2015 02:39 AM, Richard Sandiford wrote:
>
> It sounds like Jeff has a much more radical rewrite in mind,
Certainly not anything on the immediate horizon.  The amount of block 
copying that'd be needed to isolate the jump threading path would be 
significant.

I do wonder if we should be looking at a way to mark paths which have 
jump threading opportunities, but which we do not optimize and exploit 
that information in the uninit and other analysis.

Bodik had a paper on those concepts as well.  He was mostly looking at 
how to account for those paths in code coverage analysis, but there may 
be something useful in there.

> so for now how about just turning -Wmaybe-uninitialized into
> a warning for this function?  The patch will mean that it becomes
> a warning even if someone turns off warnings on the command line,
> but I don't think that's important.
>
> Bootstrapped and regression-tested on x86_64-linux-gnu.  Also tested
> with a cross-compiler to sparc-linux-gnu (which also triggered the
> warning for me).  Tested that clang could still compile the file.
> OK to install?
>
>
> gcc/
> 	* reload1.c (elimination_costs_in_insn): Locally turn
> 	-Wmaybe-uninitialized into a warning.
I can live with this.  Though I'd appreciate it if someone could reduce 
the sparcv9 testcase and create a bug to track it too.

jeff

Re: Fix reload1.c warning for some targets

[Richard Sandiford]

Jeff Law <law@redhat.com> writes:
> On 09/03/2015 02:39 AM, Richard Sandiford wrote:
>> so for now how about just turning -Wmaybe-uninitialized into
>> a warning for this function?  The patch will mean that it becomes
>> a warning even if someone turns off warnings on the command line,
>> but I don't think that's important.
>>
>> Bootstrapped and regression-tested on x86_64-linux-gnu.  Also tested
>> with a cross-compiler to sparc-linux-gnu (which also triggered the
>> warning for me).  Tested that clang could still compile the file.
>> OK to install?
>>
>>
>> gcc/
>> 	* reload1.c (elimination_costs_in_insn): Locally turn
>> 	-Wmaybe-uninitialized into a warning.
> I can live with this.  Though I'd appreciate it if someone could reduce 
> the sparcv9 testcase and create a bug to track it too.

Applied, thanks.

Richard