Hi Guys,
Long time, no post. Sorry about that. So, in order to make up for
it, here is an extra long update on the state of the GNU toolchain:
In GCC land, 6.2 has now been released and GCC 7 is almost ready to
branch. Here are the new features that you can expect to find in
GCC 7:
* LRA is now the default register allocator for new targets and most
of the major targets now use it.
* Suggestions for corrections to misspelt identifiers has been
expanded to include function names, macro names, enum values, type
names and preprocessor directives.
* The output from -fverbose-asm has been extended to add comments
showing the source lines that correspond to the assembly. For
example:
# example.c:4: int total = 0;
xorl %eax, %eax # <retval>
# example.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# example.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #
# example.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# example.c:6: for (i = 0; i < n; i++)
incl %edx # i
# example.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
* Attributes can now be set on null statements. For example:
case 1:
bar (1);
__attribute__((fallthrough));
case 2:
(The fallthrough attribute is also new. Its use should be
obvious).
* Built-in functions have been added to allow access to the PowerPC
ISA 3.0 instructions.
* Experimental support for C++17 has been added. This includes:
+ A command line option -faligned-new enables support for
allocating memory for types that require more alignment than
void*::operator new(std::size_t) provides. A numeric argument
such as -faligned-new=32 can be used to specify how much
alignment (in bytes) is provided by that function, but few users
will need to override the default of alignof(std::max_align_t).
+ The option -fnew-inheriting-ctors which enables the P0136
adjustment to the semantics of C++11 constructor inheritance.
* Some new warnings have been added as well:
+ -Wregister which warns when the register keyword is used as a
storage class specifier, (except when it is part of the GNU
Explicit Register Variables extension). The use of the
register keyword as storage class specifier has been deprecated
in C++11 and removed in C++17.
+ -Wformat-length (or -Wformat-length=<level>) warns about calls
to formatted input/output functions such as sprintf that might
overflow the destination buffer, or about bounded functions such
as snprintf that might result in output truncation.
When the exact number of bytes written by a format directive
cannot be determined at compile-time it is estimated based on
heuristics that depend on the <level> argument and on the
optimization level in use.
-Wformat-length=1 employs a conservative approach that warns
only about calls that most likely overflow the buffer or result
in output truncation. For example, this call to sprintf below
would be diagnosed:
extern int a, b;
char buf [12];
sprintf (buf, "a = %i, b = %i\n", a, b);
Increasing the buffer size by 1 however would stop the warning
from being generated as it is possible that overflow will not
occur if both a and b are small.
-Wformat-length=2 also warns about calls where numeric arguments
with maximal values would overflow. So in the example above the
warning will be generated unless the buffer is at least 34 bytes
long in order to allow for both a and b having a value of
INT_MAX.
+ -Wimplicit-fallthrough (or -Wimplicit-fallthrough=<level>) warns
when a switch case falls through. For example:
switch (cond)
{
case 1: a = 1; break;
case 2: a = 2;
case 3: a = 3; break;
}
This warning does not warn when the last statement of a case
cannot fall through, e.g. when there is a return statement or a
call to function declared with the noreturn attribute.
Since there are occasions where a switch case fall through is
desirable, GCC provides an attribute, __attribute__((fallthrough)),
that is to be used along with a null statement to suppress this
warning that would normally occur. C++17 also provides a
standard way to suppress the -Wimplicit-fallthrough warning
using [[fallthrough]]; instead of the GNU attribute.
Instead of the these attributes, it is also possible to add a
fallthrough comment to silence the warning. The whole body of
the C or C++ style comment should match the given regular
expressions listed below. The option argument <level> specifies
what kind of comments are accepted:
. -Wimplicit-fallthrough=0 disables the warning altogether.
. -Wimplicit-fallthrough=1 matches ".*" regular expression. i.e.
any comment is used as fallthrough comment.
. -Wimplicit-fallthrough=2 case insensitively matches the
".*falls?[ \t-]*thr(ough|u).*" regular expression.
. -Wimplicit-fallthrough=3 case sensitively matches one of the
following regular expressions:
-fallthrough
@fallthrough@
lint -fallthrough[ \t]*
[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?@*FALL(S | |-)?THR(OUGH|U)[
\t.!]*(-[^\n\r]*)?
[ \t.!]*(Else,? |Intentional(ly)? )?@*Fall((s | |-)[Tt]|t)hr(ough|u)[
\t.!]*(-[^\n\r]*)?
[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?@*fall(s | |-)?thr(ough|u)[
\t.!]*(-[^\n\r]*)?
. -Wimplicit-fallthrough=4 case sensitively matches one of the
following regular expressions:
-fallthrough
@fallthrough@
lint -fallthrough[ \t]*
[ \t]*FALLTHR(OUGH|U)[ \t]*
. -Wimplicit-fallthrough=5 doesn't recognize any comments as
fallthrough comments, only attributes disable the warning.
+ -Walloca warns when alloca is used anywhere in the source.
+ -Walloca-larger-than=<N> warns when a call to alloca is
unbounded or greater than <N>. Note that even seemingly
correct code involving signed integers can cause a warning.
For example:
extern signed int n;
if (n < 500) p = alloca (n);
This option also warns when alloca is used in a loop.
+ -Wvla-larger-than=<N> warns on uses of variable-length arrays
where the size is either unbounded, or bounded by an argument
that can be larger than <N> bytes. Note that GCC may optimize
small variable-length arrays of a known value into plain arrays,
so this warning may not get triggered for such arrays.
These last two warnings are not enabled by -Wall, and are only
active when the -ftree-vrp optimization is active (which by
default is when -O2 or higher is in use).
+ -Wbool-operation warns about suspicious operations on
expressions of a boolean type. For instance, bitwise negation
of a boolean is very likely a bug in the program. For C, this
warning also warns about incrementing or decrementing a boolean,
which rarely makes sense.
+ -Waligned-new warns about a new-expression of a type that
requires greater alignment than the alignof(std::max_align_t)
but which uses an allocation function without an explicit
alignment parameter.
+ -Wint-in-bool-context warns about suspicious use of integer
values where boolean values are expected, such as conditional
expressions (?:) using non-boolean integer constants in boolean
context:
if (a <= b ? 2 : 3)
Or left shifting of signed integers in a boolean context;
for (a = 0; 1 << a; a++)
* Some new optimizations have been added as well:
+ -fstore-merging This combines narrow stores to consecutive
memory addresses into larger, word sized stores.
+ -fsplit-loops splits a loop into two if it contains a condition
that's always true for one side of the iteration space and false
for the other.
+ -fprintf-return-value substitutes constants for known return
values of formatted output functions such as sprintf etc. This
transformation allows GCC to optimize or even eliminate branches
based on the known return value of these functions. For
example, both the branch and the body of the if statement (but
not the call to snprintf) can be optimized away in this snippet,
provided that the variable "i" is a 32-bit or smaller integer.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
* Lastly, but certainly not leastly, the ARM backend has added the
option -mpure-code which stops constant data from being placed in
code sections. Additionally, when compiling for ELF object format
it gives all code sections the ELF processor-specific section
attribute SHF_ARM_PURECODE.
A new release of GLIBC is unlikely to happen before the new year, but
in the meantime lots of new features have been added to the
development sources:
* Support for the ISO/IEC TS 18661-1:2014 has been added including:
- Adding the fesetexcept function to libm.
- New <math.h> features:
. Signalling NaN macros: SNANF, SNAN, SNANL.
. Comparison macros: iseqsig.
. Classification macros: iscanonical, issubnormal, iszero.
. Total order functions: totalorder, totalorderf, totalorderl,
totalordermag, totalordermagf, totalordermagl.
. Canonicalize functions: canonicalize, canonicalizef, canonicalizel.
. NaN functions: getpayload, getpayloadf, getpayloadl.
- New libc functions: strfromd, strfromf, and strfroml.
* The <sys/quota.h> header now includes the <linux/quota.h> header.
Support for the Linux quota interface which pre-dates kernel
version 2.4.22 has been removed.
* The malloc_get_state and malloc_set_state functions have been removed.
Already-existing binaries that dynamically link to these functions
will get a hidden implementation in which malloc_get_state is a
stub.
* The “ip6-dotint” and “no-ip6-dotint” resolver options, and the
corresponding RES_NOIP6DOTINT flag from <resolv.h> have been removed.
“no-ip6-dotint” had already been the default, and support for the
“ip6-dotint” option was removed from the Internet in 2006.
* The "ip6-bytestring" resolver option and the corresponding
RES_NOIP6DOTINT flag from <resolv.h> have been removed.
* DNSSEC-related declarations and definitions have been removed from
the <arpa/nameser.h> header file, and libresolv will no longer
attempt to decode the data part of DNSSEC record types.
* The resource record type classification macros ns_t_qt_p,
ns_t_mrr_p, ns_t_rr_p, ns_t_udp_p, ns_t_xfr_p have been removed
from the <arpa/nameser.h> header file because the distinction
between RR types and meta-RR types is not officially standardized,
subject to revision, and thus not suitable for encoding in a macro.
* The types res_sendhookact, res_send_qhook, re_send_rhook, and the
qhook and rhook members of the res_state type in <resolv.h> have
been removed.
GDB 7.12 is now out (really this time). In addition the development
sources has a nice new feature for MS-Windows developers:
* Native debugging on MS-Windows supports command-line redirection
Command-line arguments used for starting programs on MS-Windows
can now include redirection symbols supported by native Windows
shells, such as '<', '>', '>>', '2>&1', etc. This affects GDB
commands such as "run", "start", and "set args", as well as the
corresponding MI features.
And lastly the binutils development sources have a couple of new
tricks up its sleaves:
* Support has been added for the RISC-V architecture.
* The nm program has a new command lien option
(--with-version-strings) which will display a symbol's version
information, if any, after the symbol's name.
* The EXCLUDE_FILE linker script construct can now be applied
outside of the section list in order for the exclusions to apply
over all input sections in the list.
* The command line option --no-eh-frame-hdr can now be used in ELF
based linkers to disable the automatic generation of .eh_frame_hdr
sections.
* The ARM linker now supports --in-implib=<infile> to enable
specifying a set of Secure Gateway veneers that must exist in the
output import library specified by --out-implib=<outfile> and the
address they must have.
That's all for now. Hopefully the next update will be a bit sooner in
arriving.
Cheers
Nick
