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Re: [PATCH] Fixing improper conversion from sin() to sinf() in optimization mode.


On Fri, Sep 6, 2013 at 3:24 PM, Cong Hou <congh@google.com> wrote:
> First, thank you for your detailed comments again! Then I deeply
> apologize for not explaining my patch properly and responding to your
> previous comment. I didn't understand thoroughly the problem before
> submitting the patch.
>
> Previously I only considered the following three conversions for sqrt():
>
>
> 1: (float) sqrt ((double) float_val)  ->  sqrtf (float_val)
> 2: (float) sqrtl ((long double) float_val)  ->  sqrtf (float_val)
> 3: (double) sqrtl ((long double) double_val)  ->  sqrt (double_val)
>
>
> We have four types here:
>
> TYPE is the type to which the result of the function call is converted.
> ITYPE is the type of the math call expression.
> TREE_TYPE(arg0) is the type of the function argument (before type conversion).
> NEWTYPE is chosen from TYPE and TREE_TYPE(arg0) with higher precision.
> It will be the type of the new math call expression after conversion.
>
> For all three cases above, TYPE is always the same as NEWTYPE. That is
> why I only considered TYPE during the precision comparison. ITYPE can
> only be double_type_node or long_double_type_node depending on the
> type of the math function. That is why I explicitly used those two
> types instead of ITYPE (no correctness issue). But you are right,
> ITYPE is more elegant and better here.
>
> After further analysis, I found I missed two more cases. Note that we
> have the following conditions according to the code in convert.c:
>
> TYPE_PRECISION(NEWTYPE) >= TYPE_PRECISION(TYPE)
> TYPE_PRECISION(NEWTYPE) >= TYPE_PRECISION(TREE_TYPE(arg0))
> TYPE_PRECISION (NEWTYPE) < TYPE_PRECISION (ITYPE)
>
> the last condition comes from the fact that we only consider
> converting a math function call into another one with narrower type.
> Therefore we have
>
> TYPE_PRECISION(TYPE) < TYPE_PRECISION (ITYPE)
> TYPE_PRECISION(TREE_TYPE(arg0)) < TYPE_PRECISION (ITYPE)
>
> So for sqrt(), TYPE and TREE_TYPE(arg0) can only be float, and for
> sqrtl(), TYPE and TREE_TYPE(arg0) can be either float or double with
> four possible combinations. Therefore we have two more conversions to
> consider besides the three ones I mentioned above:
>
>
> 4: (float) sqrtl ((long double) double_val)  ->  (float) sqrt (double_val)
> 5: (double) sqrtl ((long double) float_val)  ->  sqrt ((double) float_val)
>
>
> For the first conversion here, TYPE (float) is different from NEWTYPE
> (double), and my previous patch doesn't handle this case.The correct
> way is to compare precisions of ITYPE and NEWTYPE now.
>
> To sum up, we are converting the expression
>
> (TYPE) sqrtITYPE ((ITYPE) expr)
>
> to
>
> (TYPE) sqrtNEWTYPE ((NEWTYPE) expr)
>
> and we require
>
> PRECISION (ITYPE) >= PRECISION (NEWTYPE) * 2 + 2
>
> to make it a safe conversion.
>
>
> The new patch is pasted below.
>
> I appreciate your detailed comments and analysis, and next time when I
> submit a patch I will be more carefully about the reviewer's comment.
>
>
> Thank you!
>
> Cong
>
>
>
> Index: gcc/convert.c
> ===================================================================
> --- gcc/convert.c (revision 201891)
> +++ gcc/convert.c (working copy)
> @@ -135,16 +135,19 @@ convert_to_real (tree type, tree expr)
>    CASE_MATHFN (COS)
>    CASE_MATHFN (ERF)
>    CASE_MATHFN (ERFC)
> -  CASE_MATHFN (FABS)
>    CASE_MATHFN (LOG)
>    CASE_MATHFN (LOG10)
>    CASE_MATHFN (LOG2)
>    CASE_MATHFN (LOG1P)
> -  CASE_MATHFN (LOGB)
>    CASE_MATHFN (SIN)
> -  CASE_MATHFN (SQRT)
>    CASE_MATHFN (TAN)
>    CASE_MATHFN (TANH)
> +    /* The above functions are not safe to do this conversion. */
> +    if (!flag_unsafe_math_optimizations)
> +      break;
> +  CASE_MATHFN (SQRT)
> +  CASE_MATHFN (FABS)
> +  CASE_MATHFN (LOGB)
>  #undef CASE_MATHFN
>      {
>        tree arg0 = strip_float_extensions (CALL_EXPR_ARG (expr, 0));
> @@ -155,6 +158,27 @@ convert_to_real (tree type, tree expr)
>        if (TYPE_PRECISION (TREE_TYPE (arg0)) > TYPE_PRECISION (type))
>   newtype = TREE_TYPE (arg0);
>
> +      /* We consider to convert
> +
> +     (T1) sqrtT2 ((T2) exprT3)
> + to
> +     (T1) sqrtT4 ((T4) exprT3)

Should this be

          (T4) sqrtT4 ((T4) exprT3)
> +
> +  , where T1 is TYPE, T2 is ITYPE, T3 is TREE_TYPE (ARG0),
> + and T4 is NEWTYPE.

NEWTYPE is also the wider one between T1 and T3.

David

> All those types are of floating point types.
> + T4 (NEWTYPE) should be narrower than T2 (ITYPE). This conversion
> + is safe only if P1 >= P2*2+2, where P1 and P2 are precisions of
> + T2 and T4. See the following URL for a reference:
> + http://stackoverflow.com/questions/9235456/determining-floating-point-square-root
> + */
> +      if (fcode == BUILT_IN_SQRT || fcode == BUILT_IN_SQRTL)
> + {
> +  int p1 = REAL_MODE_FORMAT (TYPE_MODE (itype))->p;
> +  int p2 = REAL_MODE_FORMAT (TYPE_MODE (newtype))->p;
> +  if (p1 < p2 * 2 + 2 && !flag_unsafe_math_optimizations)
> +    break;
> + }
> +
>        /* Be careful about integer to fp conversions.
>   These may overflow still.  */
>        if (FLOAT_TYPE_P (TREE_TYPE (arg0))
> Index: gcc/testsuite/gcc.c-torture/execute/20030125-1.c
> ===================================================================
> --- gcc/testsuite/gcc.c-torture/execute/20030125-1.c (revision 201891)
> +++ gcc/testsuite/gcc.c-torture/execute/20030125-1.c (working copy)
> @@ -44,11 +44,11 @@ __attribute__ ((noinline))
>  double
>  sin(double a)
>  {
> - abort ();
> + return a;
>  }
>  __attribute__ ((noinline))
>  float
>  sinf(float a)
>  {
> - return a;
> + abort ();
>  }
>
> On Wed, Sep 4, 2013 at 3:26 PM, Joseph S. Myers <joseph@codesourcery.com> wrote:
>> On Wed, 4 Sep 2013, Xinliang David Li wrote:
>>
>>> > This patch submission still fails to pay attention to various of my
>>> > comments.
>>> >
>>>
>>> If you can provide inlined comments in the patch, that will be more
>>> useful and productive.
>>
>> I have explained things several times in this thread already.  I see no
>> point in repeating things when what I would say has already been said and
>> ignored.  As far as I can tell, this latest patch submission has taken one
>> line from the message it is in response to, and largely ignored the
>> following two paragraphs (including where I explicitly say that said line
>> should not appear literally in the source code at all).  But, repeating
>> what I said before yet again:
>>
>>   (but you should be referring to the relevant types
>>
>> The patch does not do properly that.  It refers explicitly to
>> long_double_type_node and double_type_node.
>>
>>   - "type", the type
>>   being converted to, "itype", the type of the function being called in the
>>   source code, "TREE_TYPE (arg0)", the type of the argument after extensions
>>   have been removed, and "newtype", computed from those
>>
>> The patch only engages with "type".  I suspect "newtype" is what it really
>> means there when using "type".  When it uses long_double_type_node and
>> double_type_node, those should be "itype".
>>
>>   - so you should have
>>   expressions like the above with two or more of those four types, but not
>>   with long_double_type_node directly).
>>
>> See above.  The patch uses long_double_type_node directly, contrary to
>> what I explicitly said.  You are free to disagree with something I say in
>> a review - but in that case you need to reply specifically to the review
>> comment explaining your rationale for disagreeing with it.  Just ignoring
>> the comment and not mentioning the disagreement wastes the time of
>> reviewers.
>>
>>   The patch submission will need to include a proper analysis to justify to
>>   the reader why the particular inequality with particular types from those
>>   four is correct in all cases where the relevant code may be executed.
>>
>> The comments only refer to "T1" and "T2" without explaining how they
>> relate to the original expression (three types) or the variables within
>> GCC dealing with various types (four types, because newtype gets
>> involved).  I said back in
>> <http://gcc.gnu.org/ml/gcc-patches/2013-09/msg00161.html> and
>> <http://gcc.gnu.org/ml/gcc-patches/2013-08/msg01384.html> that three types
>> are involved - when I say "the patch submission needs to include its own
>> analysis for the full generality of three types", again, it's
>> inappropriate for a patch to omit such an analysis without justification.
>> The patch submission needs to include an analysis that properly explains
>> the transformation involved and the conditions under which it is safe.
>> Maybe starting along the lines of:
>>
>> We have an expression of the form (T1) sqrtT2 ((T2) exprT3), where exprT3
>> has type T3 (TREE_TYPE (ARG0)), T2 is the type of the floating-point
>> square root function being used (ITYPE), T1 is TYPE and all these types
>> are binary floating-point types.  We wish to optimize if possible to an
>> expression of the form (T1) sqrtT4 ((T4) exprT3), where T4 (NEWTYPE) is
>> narrower than T2.  (Then explain the choice of T4 and the conditions under
>> which the transformation is safe, with appropriate references.)
>>
>> I suggest that for the next patch submission you (the patch submitter)
>> make sure you spend at least an hour properly understanding the issues and
>> all the previous messages in the thread and writing up the detailed,
>> coherent explanation of the transformation and analysis of the issues that
>> I asked for some time ago, as if for a reader who hasn't read any of this
>> thread or looked at this transformation in GCC before.  I've certainly
>> spent longer than that on review in this thread.  It's normal for a patch
>> involving anything at all tricky to take hours to write up (and also
>> normal for one to discover, in the course of writing the detailed coherent
>> analysis for people who aren't familiar with the code and the issues
>> involved, that there's in fact something wrong with the patch and it needs
>> revisiting before submission).
>>
>> --
>> Joseph S. Myers
>> joseph@codesourcery.com


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