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

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

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