Re: [PATCH v2] BaseTools/GCC: set -Wno-unused-const-variables on RELEASE builds

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 8 September 2017 at 09:48, Laszlo Ersek <lersek@redhat.com> wrote:
> On 09/08/17 09:01, Thomas Lamprecht wrote:
>> TianoCore BZ#700 [1]
>>
>> Set the '-Wno-unused-const-variables' in RELEASE builds with the
>> GGC49 and GCC5 toolchain.
>>
>> This fixes the RELEASE build of OVMF with GCC in version 6 or newer.
>> GCC 6 added the '-Wunused-const-variable' warning, which gets
>> activated by '-Wunused-variable' and has the following behavior:
>> "Warn whenever a constant static variable is unused aside from its
>> declaration" [2]
>>
>> Commit 2ad6ba80a1bd58382bde6b994070f7c01d2fb48d introduced a case
>> where exactly this happens on a RELEASE build. All uses of the static
>> const variable are located in debug code only, which gets thrown out
>> by the compiler on RELEASE builds and thus triggers the
>> unused-const-variable warning.
>>
>> There is currently no GCC 6 toolchain target defined and doing so
>> would add a lot of boilerplate code. Instead, use the fact that GCC
>> ignores unknown '-Wno-*' options:
>>
>> "[...] if the -Wno- form is used [...] no diagnostic is produced for
>> -Wno-unknown-warning unless other diagnostics are being produced"
>>
>> This behavior is available in GCC 4.9 [3] (and also earlier, for that
>> matter), so add the flag to the GCC49 and GCC5 toolchain, even if
>> both GCC versions do not supports it.
>> GCC49 doesn't enables LTO whereas GCC5 does. As GCC 6.0 through 6.2
>> had bugs relating to LTO there can be desire to use the GCC49 target
>> even if compiling with GCC 6, see 432f1d83f77a.
>>
>> Orient the changes on 20d00edf21d2 which moved the
>> '-Wno-unused-but-set-variable' flag to RELEASE builds only, as there
>> it ensure that it does not gets raised if the only usage of a
>> variable is in (then collapsed) debug code.
>>
>> [1] https://bugzilla.tianocore.org/show_bug.cgi?id=700
>> [2] https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Warning-Options.html#index-Wunused-const-variable
>> [3] https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Warning-Options.html
>>
>> Cc: Yonghong Zhu <yonghong.zhu@intel.com>
>> Cc: Liming Gao <liming.gao@intel.com>
>> Cc: Laszlo Ersek <lersek@redhat.com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>


>> ---
>>
>> changes v1 -> v2:
>>  * add the flag also to the GCC49 toolchain to allow using GCC 6 with
>>    LTO disabled builds, thanks for the hints Laszlo and Liming!
>>  * adapt commit message slightly, especially to the fact that GCC49
>>    gets the flag now too.
>>  * CC Ard
>>
>>  BaseTools/Conf/tools_def.template | 16 ++++++++--------
>>  1 file changed, 8 insertions(+), 8 deletions(-)
>>
>> diff --git a/BaseTools/Conf/tools_def.template b/BaseTools/Conf/tools_def.template
>> index ba1d1a16de..cbb5024c1b 100755
>> --- a/BaseTools/Conf/tools_def.template
>> +++ b/BaseTools/Conf/tools_def.template
>> @@ -5179,7 +5179,7 @@ RELEASE_GCC48_AARCH64_CC_FLAGS   = DEF(GCC48_AARCH64_CC_FLAGS) -Wno-unused-but-s
>>  *_GCC49_IA32_NASM_FLAGS           = -f elf32
>>
>>    DEBUG_GCC49_IA32_CC_FLAGS       = DEF(GCC49_IA32_CC_FLAGS) -Os
>> -RELEASE_GCC49_IA32_CC_FLAGS       = DEF(GCC49_IA32_CC_FLAGS) -Os -Wno-unused-but-set-variable
>> +RELEASE_GCC49_IA32_CC_FLAGS       = DEF(GCC49_IA32_CC_FLAGS) -Os -Wno-unused-but-set-variable -Wno-unused-const-variable
>>    NOOPT_GCC49_IA32_CC_FLAGS       = DEF(GCC49_IA32_CC_FLAGS) -O0
>>
>>  ##################
>> @@ -5207,7 +5207,7 @@ RELEASE_GCC49_IA32_CC_FLAGS       = DEF(GCC49_IA32_CC_FLAGS) -Os -Wno-unused-but
>>  *_GCC49_X64_NASM_FLAGS           = -f elf64
>>
>>    DEBUG_GCC49_X64_CC_FLAGS       = DEF(GCC49_X64_CC_FLAGS) -Os
>> -RELEASE_GCC49_X64_CC_FLAGS       = DEF(GCC49_X64_CC_FLAGS) -Os -Wno-unused-but-set-variable
>> +RELEASE_GCC49_X64_CC_FLAGS       = DEF(GCC49_X64_CC_FLAGS) -Os -Wno-unused-but-set-variable -Wno-unused-const-variable
>>    NOOPT_GCC49_X64_CC_FLAGS       = DEF(GCC49_X64_CC_FLAGS) -O0
>>
>>  ##################
>> @@ -5240,7 +5240,7 @@ RELEASE_GCC49_X64_CC_FLAGS       = DEF(GCC49_X64_CC_FLAGS) -Os -Wno-unused-but-s
>>  *_GCC49_ARM_CC_XIPFLAGS          = DEF(GCC49_ARM_CC_XIPFLAGS)
>>
>>    DEBUG_GCC49_ARM_CC_FLAGS       = DEF(GCC49_ARM_CC_FLAGS) -O0
>> -RELEASE_GCC49_ARM_CC_FLAGS       = DEF(GCC49_ARM_CC_FLAGS) -Wno-unused-but-set-variable
>> +RELEASE_GCC49_ARM_CC_FLAGS       = DEF(GCC49_ARM_CC_FLAGS) -Wno-unused-but-set-variable -Wno-unused-const-variable
>>    NOOPT_GCC49_ARM_CC_FLAGS       = DEF(GCC49_ARM_CC_FLAGS) -O0
>>
>>  ##################
>> @@ -5272,7 +5272,7 @@ RELEASE_GCC49_ARM_CC_FLAGS       = DEF(GCC49_ARM_CC_FLAGS) -Wno-unused-but-set-v
>>    DEBUG_GCC49_AARCH64_DLINK_FLAGS  = DEF(GCC49_AARCH64_DLINK_FLAGS) -z common-page-size=0x1000
>>    DEBUG_GCC49_AARCH64_DLINK_XIPFLAGS = -z common-page-size=0x20
>>
>> -RELEASE_GCC49_AARCH64_CC_FLAGS     = DEF(GCC49_AARCH64_CC_FLAGS) -Wno-unused-but-set-variable -mcmodel=tiny -fomit-frame-pointer
>> +RELEASE_GCC49_AARCH64_CC_FLAGS     = DEF(GCC49_AARCH64_CC_FLAGS) -Wno-unused-but-set-variable -Wno-unused-const-variable -mcmodel=tiny -fomit-frame-pointer
>>  RELEASE_GCC49_AARCH64_DLINK_FLAGS  = DEF(GCC49_AARCH64_DLINK_FLAGS)
>>
>>    NOOPT_GCC49_AARCH64_CC_FLAGS     = DEF(GCC49_AARCH64_CC_FLAGS) -O0 -mcmodel=small
>> @@ -5326,7 +5326,7 @@ RELEASE_GCC49_AARCH64_DLINK_FLAGS  = DEF(GCC49_AARCH64_DLINK_FLAGS)
>>    DEBUG_GCC5_IA32_CC_FLAGS       = DEF(GCC5_IA32_CC_FLAGS) -flto -Os
>>    DEBUG_GCC5_IA32_DLINK_FLAGS    = DEF(GCC5_IA32_X64_DLINK_FLAGS) -flto -Os -Wl,-m,elf_i386,--oformat=elf32-i386
>>
>> -RELEASE_GCC5_IA32_CC_FLAGS       = DEF(GCC5_IA32_CC_FLAGS) -flto -Os -Wno-unused-but-set-variable
>> +RELEASE_GCC5_IA32_CC_FLAGS       = DEF(GCC5_IA32_CC_FLAGS) -flto -Os -Wno-unused-but-set-variable -Wno-unused-const-variable
>>  RELEASE_GCC5_IA32_DLINK_FLAGS    = DEF(GCC5_IA32_X64_DLINK_FLAGS) -flto -Os -Wl,-m,elf_i386,--oformat=elf32-i386
>>
>>    NOOPT_GCC5_IA32_CC_FLAGS       = DEF(GCC5_IA32_CC_FLAGS) -O0
>> @@ -5358,7 +5358,7 @@ RELEASE_GCC5_IA32_DLINK_FLAGS    = DEF(GCC5_IA32_X64_DLINK_FLAGS) -flto -Os -Wl,
>>    DEBUG_GCC5_X64_CC_FLAGS        = DEF(GCC5_X64_CC_FLAGS) -flto -DUSING_LTO -Os
>>    DEBUG_GCC5_X64_DLINK_FLAGS     = DEF(GCC5_X64_DLINK_FLAGS) -flto -Os
>>
>> -RELEASE_GCC5_X64_CC_FLAGS        = DEF(GCC5_X64_CC_FLAGS) -flto -DUSING_LTO -Os -Wno-unused-but-set-variable
>> +RELEASE_GCC5_X64_CC_FLAGS        = DEF(GCC5_X64_CC_FLAGS) -flto -DUSING_LTO -Os -Wno-unused-but-set-variable -Wno-unused-const-variable
>>  RELEASE_GCC5_X64_DLINK_FLAGS     = DEF(GCC5_X64_DLINK_FLAGS) -flto -Os
>>
>>    NOOPT_GCC5_X64_CC_FLAGS        = DEF(GCC5_X64_CC_FLAGS) -O0
>> @@ -5393,7 +5393,7 @@ RELEASE_GCC5_X64_DLINK_FLAGS     = DEF(GCC5_X64_DLINK_FLAGS) -flto -Os
>>    DEBUG_GCC5_ARM_CC_FLAGS        = DEF(GCC5_ARM_CC_FLAGS) -O0
>>    DEBUG_GCC5_ARM_DLINK_FLAGS     = DEF(GCC5_ARM_DLINK_FLAGS)
>>
>> -RELEASE_GCC5_ARM_CC_FLAGS        = DEF(GCC5_ARM_CC_FLAGS) -flto -Wno-unused-but-set-variable
>> +RELEASE_GCC5_ARM_CC_FLAGS        = DEF(GCC5_ARM_CC_FLAGS) -flto -Wno-unused-but-set-variable -Wno-unused-const-variable
>>  RELEASE_GCC5_ARM_DLINK_FLAGS     = DEF(GCC5_ARM_DLINK_FLAGS) -flto -Os -L$(WORKSPACE)/ArmPkg/Library/GccLto -llto-arm -Wl,-plugin-opt=-pass-through=-llto-arm
>>
>>    NOOPT_GCC5_ARM_CC_FLAGS        = DEF(GCC5_ARM_CC_FLAGS) -O0
>> @@ -5428,7 +5428,7 @@ RELEASE_GCC5_ARM_DLINK_FLAGS     = DEF(GCC5_ARM_DLINK_FLAGS) -flto -Os -L$(WORKS
>>    DEBUG_GCC5_AARCH64_DLINK_FLAGS = DEF(GCC5_AARCH64_DLINK_FLAGS) -z common-page-size=0x1000
>>    DEBUG_GCC5_AARCH64_DLINK_XIPFLAGS = -z common-page-size=0x20
>>
>> -RELEASE_GCC5_AARCH64_CC_FLAGS    = DEF(GCC5_AARCH64_CC_FLAGS) -flto -Wno-unused-but-set-variable -mcmodel=tiny -fomit-frame-pointer
>> +RELEASE_GCC5_AARCH64_CC_FLAGS    = DEF(GCC5_AARCH64_CC_FLAGS) -flto -Wno-unused-but-set-variable -Wno-unused-const-variable -mcmodel=tiny -fomit-frame-pointer
>>  RELEASE_GCC5_AARCH64_DLINK_FLAGS = DEF(GCC5_AARCH64_DLINK_FLAGS) -flto -Os -L$(WORKSPACE)/ArmPkg/Library/GccLto -llto-aarch64 -Wl,-plugin-opt=-pass-through=-llto-aarch64 -Wno-lto-type-mismatch
>>
>>    NOOPT_GCC5_AARCH64_CC_FLAGS    = DEF(GCC5_AARCH64_CC_FLAGS) -O0 -mcmodel=small
>>
>
>
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>