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

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

Reviewed-by: Liming Gao <liming.gao@intel.com>

>-----Original Message-----
>From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of
>Thomas Lamprecht
>Sent: Friday, September 08, 2017 3:02 PM
>To: edk2-devel@lists.01.org
>Cc: Laszlo Ersek <lersek@redhat.com>; Gao, Liming <liming.gao@intel.com>;
>Ard Biesheuvel <ard.biesheuvel@linaro.org>
>Subject: [edk2] [PATCH v2] BaseTools/GCC: set -Wno-unused-const-variables
>on RELEASE builds
>
>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>
>---
>
>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
>--
>2.11.0
>
>
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