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[Ada] Generate warning for negative literal of a modular type

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gcc/ada/

	* opt.ads: Update comment for Warn_On_Suspicious_Modulus_Value.
	* sem_res.adb (Resolve_Unary_Op): Generate warning.
	* usage.adb: Refine doc for -gnatw.m/M switch.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Update doc on -gnatw.m switch.
	* gnat_ugn.texi: Regenerate.
This commit is contained in:
Yannick Moy 2021-02-02 12:31:04 +01:00 • committed by Pierre-Marie de Rodat
parent bf62c9012c
commit 3a700c5fef
5 changed files with 36 additions and 6 deletions
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5
gcc/ada/doc/gnat_ugn/building_executable_programs_with_gnat.rst
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@ -3424,7 +3424,10 @@ of the pragma in the :title:`GNAT_Reference_manual`).
with no size clause. The guess in both cases is that 2**x was intended
rather than x. In addition expressions of the form 2*x for small x
generate a warning (the almost certainly accurate guess being that
2**x was intended). The default is that these warnings are given.
2**x was intended). This switch also activates warnings for negative
literal values of a modular type, which are interpreted as large positive
integers after wrap-around. The default is that these warnings are given.
.. index:: -gnatw.M (gcc)

4
gcc/ada/gnat_ugn.texi
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@ -11616,7 +11616,9 @@ a modulus of 7 with a size of 7 bits), and modulus values of 32 or 64
with no size clause. The guess in both cases is that 2**x was intended
rather than x. In addition expressions of the form 2*x for small x
generate a warning (the almost certainly accurate guess being that
2**x was intended). The default is that these warnings are given.
2**x was intended). This switch also activates warnings for negative
literal values of a modular type, which are interpreted as large positive
integers after wrap-around. The default is that these warnings are given.
@end table
@geindex -gnatw.M (gcc)

5
gcc/ada/opt.ads
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@ -1885,8 +1885,9 @@ package Opt is
Warn_On_Suspicious_Modulus_Value : Boolean := True;
-- GNAT
-- Set to True to generate warnings for suspicious modulus values. The
-- default is that this warning is enabled. Modified by -gnatw.m/.M.
-- Set to True to generate warnings for suspicious modulus values, as well
-- as negative literals of a modular type. The default is that this warning
-- is enabled. Modified by -gnatw.m/.M.
Warn_On_Unchecked_Conversion : Boolean := True;
-- GNAT

22
gcc/ada/sem_res.adb
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@ -12096,6 +12096,28 @@ package body Sem_Res is
Set_Etype (N, B_Typ);
Resolve (R, B_Typ);
-- Generate warning for negative literal of a modular type, unless it is
-- enclosed directly in a type qualification or a type conversion, as it
-- is likely not what the user intended. We don't issue the warning for
-- the common use of -1 to denote OxFFFF_FFFF...
if Warn_On_Suspicious_Modulus_Value
and then Nkind (N) = N_Op_Minus
and then Nkind (R) = N_Integer_Literal
and then Is_Modular_Integer_Type (B_Typ)
and then Nkind (Parent (N)) not in N_Qualified_Expression
| N_Type_Conversion
and then Expr_Value (R) > Uint_1
then
Error_Msg_N
("?M?negative literal of modular type is in fact positive", N);
Error_Msg_Uint_1 := (-Expr_Value (R)) mod Modulus (B_Typ);
Error_Msg_Uint_2 := Expr_Value (R);
Error_Msg_N ("\do you really mean^ when writing -^ '?", N);
Error_Msg_N
("\if you do, use qualification to avoid this warning", N);
end if;
-- Generate warning for expressions like abs (x mod 2)
if Warn_On_Redundant_Constructs

6
gcc/ada/usage.adb
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@ -532,8 +532,10 @@ begin
"but not read");
Write_Line (" M* turn off warnings for variable assigned " &
"but not read");
Write_Line (" .m*+ turn on warnings for suspicious modulus value");
Write_Line (" .M turn off warnings for suspicious modulus value");
Write_Line (" .m*+ turn on warnings for suspicious usage " &
"of modular type");
Write_Line (" .M turn off warnings for suspicious usage " &
"of modular type");
Write_Line (" n* normal warning mode (cancels -gnatws/-gnatwe)");
Write_Line (" .n turn on info messages for atomic " &
"synchronization");