c/c++: new warning: -Wxor-used-as-pow [PR90885]

c/c++: new warning: -Wxor-used-as-pow [PR90885]

PR c/90885 notes various places in real-world code where people have
written C/C++ code that uses ^ (exclusive or) where presumbably they
meant exponentiation.

For example
  https://codesearch.isocpp.org/cgi-bin/cgi_ppsearch?q=2%5E32&search=Search
currently finds 11 places using "2^32", and all of them appear to be
places where the user means 2 to the power of 32, rather than 2
exclusive-orred with 32 (which is 34).

This patch adds a new -Wxor-used-as-pow warning to the C and C++
frontends to complain about ^ when the left-hand side is the decimal
constant 2 or the decimal constant 10.

This is the same name as the corresponding clang warning:
  https://clang.llvm.org/docs/DiagnosticsReference.html#wxor-used-as-pow

As per the clang warning, the warning suggests converting the left-hand
side to a hexadecimal constant if you really mean xor, which suppresses
the warning (though this patch implements a fix-it hint for that, whereas
the clang implementation only has a fix-it hint for the initial
suggestion of exponentiation).

I initially tried implementing this without checking for decimals, but
this version had lots of false positives.  Checking for decimals
requires extending the lexer to capture whether or not a CPP_NUMBER
token was decimal.  I added a new DECIMAL_INT flag to cpplib.h for this.
Unfortunately, c_token and cp_tokens both have only an unsigned char for
their flags (as captured by c_lex_with_flags), whereas this would add
the 12th flag to cpp_tokens.  Of the first 8 flags, all but BOL are used
in the C or C++ frontends, but BOL is not, so I moved that to a higher
position, using its old value for the new DECIMAL_INT flag, so that it
is representable within an unsigned char.

Example output:

demo.c:5:13: warning: result of '2^8' is 10; did you mean '1 << 8' (256)? [-Wxor-used-as-pow]
    5 | int t2_8 = 2^8;
      |             ^
      |            --
      |            1<<
demo.c:5:12: note: you can silence this warning by using a hexadecimal constant (0x2 rather than 2)
    5 | int t2_8 = 2^8;
      |            ^
      |            0x2
demo.c:21:15: warning: result of '10^6' is 12; did you mean '1e6'? [-Wxor-used-as-pow]
   21 | int t10_6 = 10^6;
      |               ^
      |             ---
      |             1e
demo.c:21:13: note: you can silence this warning by using a hexadecimal constant (0xa rather than 10)
   21 | int t10_6 = 10^6;
      |             ^~
      |             0xa

gcc/c-family/ChangeLog:
	PR c/90885
	* c-common.h (check_for_xor_used_as_pow): New decl.
	* c-lex.cc (c_lex_with_flags): Add DECIMAL_INT to flags as appropriate.
	* c-warn.cc (check_for_xor_used_as_pow): New.
	* c.opt (Wxor-used-as-pow): New.

gcc/c/ChangeLog:
	PR c/90885
	* c-parser.cc (c_parser_string_literal): Clear ret.m_decimal.
	(c_parser_expr_no_commas): Likewise.
	(c_parser_conditional_expression): Likewise.
	(c_parser_binary_expression): Clear m_decimal when popping the
	stack.
	(c_parser_unary_expression): Clear ret.m_decimal.
	(c_parser_has_attribute_expression): Likewise for result.
	(c_parser_predefined_identifier): Likewise for expr.
	(c_parser_postfix_expression): Likewise for expr.
	Set expr.m_decimal when handling a CPP_NUMBER that was a decimal
	token.
	* c-tree.h (c_expr::m_decimal): New bitfield.
	* c-typeck.cc (parser_build_binary_op): Clear result.m_decimal.
	(parser_build_binary_op): Call check_for_xor_used_as_pow.

gcc/cp/ChangeLog:
	PR c/90885
	* cp-tree.h (class cp_expr): Add bitfield m_decimal.  Clear it in
	existing ctors.  Add ctor that allows specifying its value.
	(cp_expr::decimal_p): New accessor.
	* parser.cc (cp_parser_expression_stack_entry::flags): New field.
	(cp_parser_primary_expression): Set m_decimal of cp_expr when
	handling numbers.
	(cp_parser_binary_expression): Extract flags from token when
	populating stack.  Call check_for_xor_used_as_pow.

gcc/ChangeLog:
	PR c/90885
	* doc/invoke.texi (Warning Options): Add -Wxor-used-as-pow.

gcc/testsuite/ChangeLog:
	PR c/90885
	* c-c++-common/Wxor-used-as-pow-1.c: New test.
	* c-c++-common/Wxor-used-as-pow-fixits.c: New test.
	* g++.dg/parse/expr3.C: Convert 2 to 0x2 to suppress
	-Wxor-used-as-pow.
	* g++.dg/warn/Wparentheses-10.C: Likewise.
	* g++.dg/warn/Wparentheses-18.C: Likewise.
	* g++.dg/warn/Wparentheses-19.C: Likewise.
	* g++.dg/warn/Wparentheses-9.C: Likewise.
	* g++.dg/warn/Wxor-used-as-pow-named-op.C: New test.
	* gcc.dg/Wparentheses-6.c: Convert 2 to 0x2 to suppress
	-Wxor-used-as-pow.
	* gcc.dg/Wparentheses-7.c: Likewise.
	* gcc.dg/precedence-1.c: Likewise.

libcpp/ChangeLog:
	PR c/90885
	* include/cpplib.h (BOL): Move macro to 1 << 12 since it is
	not used by C/C++'s unsigned char token flags.
	(DECIMAL_INT): New, using 1 << 6, so that it is visible as
	part of C/C++'s 8 bits of token flags.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>