Re: [PATCH] BaseTools/GCC5: set -Wno-unused-const-variables on RELEASE builds

["Gao, Liming" <liming.gao@intel.com>]

I suggest to also add this option in GCC49 tool chain RELEASE builds. GCC49 can be used for GCC 4.9 or the above version. It doesn't enable LTO. 

Thanks
Liming
> -----Original Message-----
> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Thomas Lamprecht
> Sent: Thursday, September 7, 2017 10:55 PM
> To: edk2-devel@lists.01.org
> Cc: Laszlo Ersek <lersek@redhat.com>; Gao, Liming <liming.gao@intel.com>
> Subject: [edk2] [PATCH] BaseTools/GCC5: set -Wno-unused-const-variables on RELEASE builds
> 
> TianoCore BZ#700 <https://bugzilla.tianocore.org/show_bug.cgi?id=700>
> 
> 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" [1]
> 
> 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 unkown '-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 5 [2] (and also earlier, for that
> matter), so add the flag to the GCC5 toolchain, even if GCC 5 itself
> does not supports it.
> 
> 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://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Warning-Options.html#index-Wunused-const-variable
> [2] https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/Warning-Options.html
> 
> Cc: Yonghong Zhu <yonghong.zhu@intel.com>
> Cc: Liming Gao <liming.gao@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
> ---
> 
> I hope I CCed the correct people and got the style right :)
> 
>  BaseTools/Conf/tools_def.template | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/BaseTools/Conf/tools_def.template b/BaseTools/Conf/tools_def.template
> index ba1d1a16de..7db7a174c3 100755
> --- a/BaseTools/Conf/tools_def.template
> +++ b/BaseTools/Conf/tools_def.template
> @@ -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
> --
> 2.11.0
> 
> 
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