Commit 0547dbb725b reduced the number of cases in which
union padding bits are zeroed when the relevant language
standard does not strictly require it, unless gcc was
invoked with -fzero-init-padding-bits=unions or
-fzero-init-padding-bits=all in order to explicitly
request zeroing of padding bits.
This commit adds a closely related warning,
-Wzero-init-padding-bits=, which is intended to help
programmers to find code that might now need to be
rewritten or recompiled with
-fzero-init-padding-bits=unions or
-fzero-init-padding-bits=all in order to replicate
the behaviour that it had when compiled by older
versions of GCC. It can also be used to find struct
padding that was never previously guaranteed to be
zero initialized and still isn't unless GCC is
invoked with -fzero-init-padding-bits=all option.
The new warning can be set to the same three states
as -fzero-init-padding-bits ('standard', 'unions'
or 'all') and has the same default value ('standard').
The two options interact as follows:
f: standard f: unions f: all
w: standard X X X
w: unions U X X
w: all A S X
X = No warnings about padding
U = Warnings about padding of unions.
S = Warnings about padding of structs.
A = Warnings about padding of structs and unions.
The level of optimisation and whether or not the
entire initializer is dropped to memory can both
affect whether warnings are produced when compiling
a given program. This is intentional, since tying
the warnings more closely to the relevant language
standard would require a very different approach
that would still be target-dependent, might impose
an unacceptable burden on programmers, and would
risk not satisfying the intended use-case (which
is closely tied to a specific optimisation).
Bootstrapped the compiler and tested on AArch64
and x86-64 using some new tests for
-Wzero-init-padding-bits and the existing tests
for -fzero-init-padding-bits
(check-gcc RUNTESTFLAGS="dg.exp=*-empty-init-*.c").
Base commit is a470433732e77ae29a717cf79049ceeea3cbe979
Changes in v2:
- Added missing changelog entry.
Changes in v3:
- Modified two tests in which I had neglected to
ensure that initializers were not compile time
constants. This policy prevents the entire
initializer being dropped to memory, which
would otherwise prevent the expected diagnostic
message from being produced.
- Amended the diagnostic message from "Padding bits
might not.." to "padding might not..."
Changes in v4:
- Removed redundant braces.
- Added "if code relies on it being zero," to the
diagnostic message.
Link to v1:
https://inbox.sourceware.org/gcc-patches/20250520104940.3546-1-chris.baz...@arm.com/
Link to v2:
https://inbox.sourceware.org/gcc-patches/20250520144524.5968-1-chris.baz...@arm.com/
Link to v3:
https://inbox.sourceware.org/gcc-patches/20250521124745.24592-1-chris.baz...@arm.com/
Christopher Bazley (1):
Add warnings of potentially-uninitialized padding bits
gcc/common.opt | 4 +
gcc/doc/invoke.texi | 85 ++++++++++++++++++-
gcc/expr.cc | 41 ++++-----
gcc/expr.h | 7 +-
gcc/gimplify.cc | 27 +++++-
gcc/testsuite/gcc.dg/c23-empty-init-warn-1.c | 68 +++++++++++++++
gcc/testsuite/gcc.dg/c23-empty-init-warn-10.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-11.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-12.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-13.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-14.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-15.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-16.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-17.c | 51 +++++++++++
gcc/testsuite/gcc.dg/c23-empty-init-warn-2.c | 69 +++++++++++++++
gcc/testsuite/gcc.dg/c23-empty-init-warn-3.c | 7 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-4.c | 69 +++++++++++++++
gcc/testsuite/gcc.dg/c23-empty-init-warn-5.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-6.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-7.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-8.c | 8 ++
gcc/testsuite/gcc.dg/c23-empty-init-warn-9.c | 69 +++++++++++++++
.../gcc.dg/gnu11-empty-init-warn-1.c | 52 ++++++++++++
.../gcc.dg/gnu11-empty-init-warn-10.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-11.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-12.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-13.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-14.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-15.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-16.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-17.c | 51 +++++++++++
.../gcc.dg/gnu11-empty-init-warn-2.c | 59 +++++++++++++
.../gcc.dg/gnu11-empty-init-warn-3.c | 7 ++
.../gcc.dg/gnu11-empty-init-warn-4.c | 63 ++++++++++++++
.../gcc.dg/gnu11-empty-init-warn-5.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-6.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-7.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-8.c | 8 ++
.../gcc.dg/gnu11-empty-init-warn-9.c | 55 ++++++++++++
39 files changed, 935 insertions(+), 25 deletions(-)
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-1.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-10.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-11.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-12.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-13.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-14.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-15.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-16.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-17.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-2.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-3.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-4.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-5.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-6.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-7.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-8.c
create mode 100644 gcc/testsuite/gcc.dg/c23-empty-init-warn-9.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-1.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-10.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-11.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-12.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-13.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-14.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-15.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-16.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-17.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-2.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-3.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-4.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-5.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-6.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-7.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-8.c
create mode 100644 gcc/testsuite/gcc.dg/gnu11-empty-init-warn-9.c
--
2.43.0
