[src/trunk]: src/external/gpl3/gcc document various gcc warnings that occur f...

[mrg <mrg%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/7662158d0190
branches:  trunk
changeset: 961307:7662158d0190
user:      mrg <mrg%NetBSD.org@localhost>
date:      Thu Apr 15 05:15:04 2021 +0000

description:
document various gcc warnings that occur frequently enough to
inspire the GCC_NO_* series of variables, and explain some
about what the issues may be for each, hopefully providing some
direction on how to fix new warnings in this space.

christos asked me for this last year, and it's finally complete
enough to commit.  :-)

diffstat:

 external/gpl3/gcc/README.warnings |  132 ++++++++++++++++++++++++++++++++++++++
 1 files changed, 132 insertions(+), 0 deletions(-)

diffs (136 lines):

diff -r 08058b899e91 -r 7662158d0190 external/gpl3/gcc/README.warnings
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/external/gpl3/gcc/README.warnings Thu Apr 15 05:15:04 2021 +0000
@@ -0,0 +1,132 @@
+$NetBSD: README.warnings,v 1.1 2021/04/15 05:15:04 mrg Exp $
+
+What to do about GCC warnings and NetBSD.
+
+
+New GCC releases always come with a host of new warnings and
+each new warning can find real bugs, find odd code, or simply
+be a pain of new useless warnings, or all three and more.
+
+As each warning has its own set of issues they each have their
+own section and it is expected that this document will be
+modified for updates to warnings and new warnings.
+
+
+<bsd.own.mk> provides several variables for use in Makefiles:
+   COPTS.foo.c += ${GCC_NO_FORMAT_TRUNCATION}
+   COPTS.foo.c += ${GCC_NO_FORMAT_OVERFLOW}
+   COPTS.foo.c += ${GCC_NO_STRINGOP_OVERFLOW}
+   COPTS.foo.c += ${GCC_NO_STRINGOP_TRUNCATION}
+   COPTS.foo.c += ${GCC_NO_CAST_FUNCTION_TYPE}
+   COPTS.foo.c += ${GCC_NO_IMPLICIT_FALLTHRU}
+   COPTS.foo.c += ${GCC_NO_ADDR_OF_PACKED_MEMBER}
+   COPTS.foo.c += ${GCC_NO_MAYBE_UNINITIALIZED}
+   COPTS.foo.c += ${GCC_NO_RETURN_LOCAL_ADDR}
+
+
+new GCC 10 warnings:
+
+  GCC 10 switched the default from "-fcommon" to "-fno-common",
+  which can cause multiply defined symbol issues.  Ideally we
+  fix all of these, but "-fcommon" can be used otherwise.
+
+  -Wno-maybe-uninitialized
+
+      This warning was introduced in an ancient GCC but was
+      significantly enhanced in GCC 10, unfortunately, many of
+      the new instances are incorrect.
+
+      bsd.own.mk variable: ${GCC_NO_MAYBE_UNINITIALIZED}
+
+  -Wno-return-local-addr
+
+      This warning was introduced in GCC 5 and was enhanced in GCC
+      10.  Unfortunately, the new instances are failing to correctly
+      analyze code flow and miss that most of the are handled.
+
+      bsd.own.mk variable: ${GCC_NO_RETURN_LOCAL_ADDR}
+
+
+new GCC 9 warnings:
+
+  -Wno-address-of-packed-member
+
+      This warning was introduced in GCC 8.
+      This warning is similar to -Wformat-truncation, but for the
+      general family of string functions (str*(), etc.), and has
+      similar issues of false positives.
+
+      bsd.own.mk variable: ${GCC_NO_ADDR_OF_PACKED_MEMBER}
+
+
+new GCC 8 warnings:
+
+  -Wstringop-truncation
+
+      This warning was introduced in GCC 8.
+      This warning is similar to -Wformat-truncation, but for the
+      general family of string functions (str*(), etc.), and has
+      similar issues of false positives.
+
+      bsd.own.mk variable: ${GCC_NO_STRINGOP_TRUNCATION}
+
+
+  -Wcast-function-type
+
+      This warning was introduced in GCC 8.
+      This warning can find real problems.  Most instances are
+      false positives, and hopefully this warning will become
+      more useful in the future.  See __FPTRCAST().
+
+      bsd.own.mk variable: ${GCC_NO_CAST_FUNCTION_TYPE}
+
+
+new GCC 7 warnings:
+
+  -Wstringop-overflow
+
+      This warning was introduced in GCC 7.
+      This warning can find issues where source length is
+      passed as destination length (eg, strncpy() where the
+      length is strlen(src)) that are potential buffer overflow
+      cases and should always be inspected, but false positives
+      are also seen.
+
+      bsd.own.mk variable: ${GCC_NO_STRINGOP_OVERFLOW}
+
+
+  -Wformat-truncation
+
+    This warning was introduced in GCC 7.
+    This warning has many false positives where truncation is
+    either expected or unavoidable, but also finds several real
+    code bugs.
+
+    Code should always be manually inspected for this warning
+    as it does pick up real issues.
+
+    bsd.own.mk variable: ${GCC_NO_FORMAT_TRUNCATION}
+
+
+  -Wformat-overflow
+
+    This warning was introduced in GCC 7.
+    This warning typically identifies a real problem, but it may
+    fail to notice the code handles this case.
+
+    Code should always be manually inspected for this warning
+    as it does pick up real issues.
+
+    bsd.own.mk variable: ${GCC_NO_FORMAT_OVERFLOW}
+
+
+  -Wimplicit-fallthrough
+
+    This warning was introduced in GCC 7.
+    This warning has many false positives in GCC 7, and many are
+    fixed in newer GCC 10.  Old uses should be checked occasionally.
+
+    Code should always be manually inspected for this warning
+    as it does pick up real issues.
+
+    bsd.own.mk variable: ${GCC_NO_IMPLICIT_FALLTHRU}