[patch, fortran] Warn about integer**(negative integer)

[patch, fortran] Warn about integer**(negative integer)

[Thomas Koenig]

[-- Attachment #1: Type: text/plain, Size: 707 bytes --]

Hello world,

a reacent thread on c.l.f showed another common error - expecting
10**(-3) to expect something different from zero.  The attached
patch warns about this result if -Winteger-division is active.

Why -Winteger-division?  Two reasons:  First, 10**(-3) ist just
a shorthand for integer division.  Second, I did not want to add
yet another warning flag.

Regression-tested.  OK for trunk?

	Thomas

2017-01-22  Thomas Koenig  <tkoenig@netcologne.de>

         * arith.c (arith_power):  If simplifying integer power expression
         to zero, warn if -Winteger-division is given.

2017-01-22  Thomas Koenig  <tkoenig@netcologne.de>

         * gfortran.dg/integer_exponentiation_7.f90:  New test.

[-- Attachment #2: p1.diff --]
[-- Type: text/x-patch, Size: 591 bytes --]

Index: arith.c
===================================================================
--- arith.c	(Revision 244747)
+++ arith.c	(Arbeitskopie)
@@ -874,6 +874,11 @@ arith_power (gfc_expr *op1, gfc_expr *op2, gfc_exp
 		  {
 		    /* if op2 < 0, op1**op2 == 0  because abs(op1) > 1.  */
 		    mpz_set_si (result->value.integer, 0);
+		    if (warn_integer_division)
+		      gfc_warning_now (OPT_Winteger_division, "Integer "
+				       "exponentiation truncated to constant "
+				       "%qs at %L", "0", &result->where);
+				       
 		  }
 		else if (gfc_extract_int (op2, &power))
 		  {

[-- Attachment #3: integer_exponentiation_7.f90 --]
[-- Type: text/x-fortran, Size: 171 bytes --]

! { dg-do compile }
! { dg-options "-Winteger-division" }
program main
  print *,10**(-3) ! { dg-warning "Integer exponentiation truncated to constant" }
end program main

Re: [patch, fortran] Warn about integer**(negative integer)

[FX]

> 2017-01-22  Thomas Koenig  <tkoenig@netcologne.de>
> 
>        * arith.c (arith_power):  If simplifying integer power expression
>        to zero, warn if -Winteger-division is given.
> 
> 2017-01-22  Thomas Koenig  <tkoenig@netcologne.de>
> 
>        * gfortran.dg/integer_exponentiation_7.f90:  New test.

The idea seems reasonable, but I am not sure about the wording (“truncated” in particular). “Integer exponentiation truncated to constant 0”? Why not “Negative integer exponent has result value zero”?

Other than that, OK to commit.

FX

Re: [patch, fortran] Warn about integer**(negative integer)

[Thomas Koenig]

Hi FX,

> The idea seems reasonable, but I am not sure about the wording
> (“truncated” in particular).  “Integer exponentiation truncated
> to constant 0”? Why not “Negative integer exponent has result
> value zero”?
>
> Other than that, OK to commit.


You're right, the error message was a bit strange.  Here is the
version of the patch I committed with corresponding change in the
test case.

Regards

	Thomas


Index: fortran/arith.c
===================================================================
--- fortran/arith.c     (Revision 244747)
+++ fortran/arith.c     (Arbeitskopie)
@@ -874,6 +874,10 @@ arith_power (gfc_expr *op1, gfc_expr *op2, gfc_exp
                   {
                     /* if op2 < 0, op1**op2 == 0  because abs(op1) > 1.  */
                     mpz_set_si (result->value.integer, 0);
+                   if (warn_integer_division)
+                     gfc_warning_now (OPT_Winteger_division, "Negative 
" 

+                                      "exponent of integer has zero "
+                                      "result at %L", &result->where);
                   }
                 else if (gfc_extract_int (op2, &power))
                   {