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

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

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