Allow clearer suppression of toolchain and compiler prefixes

Currently it is counterintuitive to disable toolchain prefixes: we need
to pass an empty space to it; passing an empty string would not disable
prefixes but instead instruct the build script to use a set of default
prefixes.

With this patch we restore the intuitive approach of passing an empty
string "" to disable the prefix.

Resolves COMPMID-5353, COMPMID-5380

Signed-off-by: SiCong Li <sicong.li@arm.com>
Change-Id: I2fe84689df9093cf6baf507dde44ca5ebf61023b
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/7686
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>

3 files changed, 44 insertions, 15 deletions

diff --git a/SConstruct b/SConstruct
index 0f5df59dae..b0b5649a5c 100644
--- a/SConstruct
+++ b/SConstruct
@@ -118,8 +118,8 @@ vars.AddVariables(
     ListVariable("custom_options", "Custom options that can be used to turn on/off features", "none", ["disable_mmla_fp"]),
     ListVariable("data_type_support", "Enable a list of data types to support", "all", ["qasymm8", "qasymm8_signed", "qsymm16", "fp16", "fp32", "integer"]),
     ListVariable("data_layout_support", "Enable a list of data layout to support", "all", ["nhwc", "nchw"]),
-    ("toolchain_prefix", "Override the toolchain prefix; used by all toolchain components: compilers, linker, assembler etc.", ""),
-    ("compiler_prefix", "Override the compiler prefix; used by just compilers (CC,CXX); further overrides toolchain_prefix for compilers; if left empty, SCons only uses toolchain_prefix; this is for when the compiler prefixes are different from that of the linkers, archivers etc.", ""),
+    ("toolchain_prefix", "Override the toolchain prefix; used by all toolchain components: compilers, linker, assembler etc. If unspecified, use default(auto) prefixes; if passed an empty string '' prefixes would be disabled", "auto"),
+    ("compiler_prefix", "Override the compiler prefix; used by just compilers (CC,CXX); further overrides toolchain_prefix for compilers; this is for when the compiler prefixes are different from that of the linkers, archivers etc. If unspecified, this is the same as toolchain_prefix; if passed an empty string '' prefixes would be disabled", "auto"),
     ("extra_cxx_flags", "Extra CXX flags to be appended to the build command", ""),
     ("extra_link_flags", "Extra LD flags to be appended to the build command", ""),
     ("compiler_cache", "Command to prefix to the C and C++ compiler (e.g ccache)", ""),
@@ -323,35 +323,42 @@ else: # NONE "multi_isa" builds
 # Define toolchain
 # The reason why we distinguish toolchain_prefix from compiler_prefix is for cases where the linkers/archivers use a
 # different prefix than the compilers. An example is the NDK r20 toolchain
-toolchain_prefix = ""
+auto_toolchain_prefix = ""
 if 'x86' not in env['arch']:
     if env['estate'] == '32':
         if env['os'] == 'linux':
-            toolchain_prefix = "arm-linux-gnueabihf-" if 'v7' in env['arch'] else "armv8l-linux-gnueabihf-"
+            auto_toolchain_prefix = "arm-linux-gnueabihf-" if 'v7' in env['arch'] else "armv8l-linux-gnueabihf-"
         elif env['os'] == 'bare_metal':
-            toolchain_prefix = "arm-eabi-"
+            auto_toolchain_prefix = "arm-eabi-"
         elif env['os'] == 'android':
-            toolchain_prefix = "arm-linux-androideabi-"
+            auto_toolchain_prefix = "arm-linux-androideabi-"
         elif env['os'] == 'tizen':
-            toolchain_prefix = "armv7l-tizen-linux-gnueabi-"
+            auto_toolchain_prefix = "armv7l-tizen-linux-gnueabi-"
     elif env['estate'] == '64' and 'v8' in env['arch']:
         if env['os'] == 'linux':
-            toolchain_prefix = "aarch64-linux-gnu-"
+            auto_toolchain_prefix = "aarch64-linux-gnu-"
         elif env['os'] == 'bare_metal':
-            toolchain_prefix = "aarch64-elf-"
+            auto_toolchain_prefix = "aarch64-elf-"
         elif env['os'] == 'android':
-            toolchain_prefix = "aarch64-linux-android-"
+            auto_toolchain_prefix = "aarch64-linux-android-"
         elif env['os'] == 'tizen':
-            toolchain_prefix = "aarch64-tizen-linux-gnu-"
+            auto_toolchain_prefix = "aarch64-tizen-linux-gnu-"
 
 if env['build'] == 'native':
     toolchain_prefix = ""
 
-if env["toolchain_prefix"] != "":
+if env["toolchain_prefix"] == "":
+    toolchain_prefix = ""
+elif env["toolchain_prefix"] == "auto":
+    toolchain_prefix = auto_toolchain_prefix
+else:
     toolchain_prefix = env["toolchain_prefix"]
 
-compiler_prefix = toolchain_prefix
-if env["compiler_prefix"] != "":
+if env["compiler_prefix"] == "":
+    compiler_prefix = ""
+elif env["compiler_prefix"] == "auto":
+    compiler_prefix = toolchain_prefix
+else:
     compiler_prefix = env["compiler_prefix"]
 
 env['CC'] = env['compiler_cache']+ " " + compiler_prefix + c_compiler
@@ -364,6 +371,9 @@ else:
     env['AR'] = toolchain_prefix + "ar"
 env['RANLIB'] = toolchain_prefix + "ranlib"
 
+print("Using compilers:")
+print("CC", env['CC'])
+print("CXX", env['CXX'])
 
 if not GetOption("help"):
     try:
diff --git a/docs/user_guide/how_to_build_and_run_examples.dox b/docs/user_guide/how_to_build_and_run_examples.dox
index ea71f6345e..ed4c6a4029 100644
--- a/docs/user_guide/how_to_build_and_run_examples.dox
+++ b/docs/user_guide/how_to_build_and_run_examples.dox
@@ -391,9 +391,21 @@ For NDK r18 or older, here is a guide to <a href="https://developer.android.com/
 	$NDK/build/tools/make_standalone_toolchain.py --arch arm --install-dir $MY_TOOLCHAINS/arm-linux-android-ndk-r18b --stl libc++ --api 21
 
 For NDK r19 or newer, you can directly <a href="https://developer.android.com/ndk/downloads">Download</a> the NDK package for your development platform, without the need to launch the make_standalone_toolchain.py script. You can find all the prebuilt binaries inside $NDK/toolchains/llvm/prebuilt/$OS_ARCH/bin/.
-@attention the building script will look for a binary named "aarch64-linux-android-clang++", while the prebuilt binaries will have their API version as a suffix to their filename (e.g. "aarch64-linux-android21-clang++"). You should copy/rename the binary removing this suffix, or - alternatively - create an alias for it.
+@parblock
+@attention The building script will look for a binary named "aarch64-linux-android-clang++", while the prebuilt binaries will have their API version as a suffix to their filename (e.g. "aarch64-linux-android21-clang++"). You can instruct scons to use the correct version by using a combination of the toolchain_prefix and the "CC" "CXX" environment variables.
+@attention For this particular example, you can specify:
 
+	CC=clang CXX=clang++ scons toolchain_prefix=aarch64-linux-android21-
+
+@attention or:
+
+	CC=aarch64-linux-android21-clang CXX=aarch64-linux-android21-clang++ scons toolchain_prefix=""
+
+@endparblock
+
+@parblock
 @attention We used to use gnustl but as of NDK r17 it is deprecated so we switched to libc++
+@endparblock
 
 @note Make sure to add the toolchains to your PATH:
 
diff --git a/docs/user_guide/release_version_and_change_log.dox b/docs/user_guide/release_version_and_change_log.dox
index 3632b27680..ac6577bae7 100644
--- a/docs/user_guide/release_version_and_change_log.dox
+++ b/docs/user_guide/release_version_and_change_log.dox
@@ -41,6 +41,13 @@ If there is more than one release in a month then an extra sequential number is
 
 @section S2_2_changelog Changelog
 
+v22.08 Public major release
+ - Build flag change: toolchain_prefix, compiler_prefix:
+   - Use empty string "" to suppress any prefixes
+   - Use "auto" to use default (auto) prefixes chosen by the build script. This is the default behavior when unspecified
+   - Any other string will be used as custom prefixes to the compiler and the rest of toolchain tools
+   - The default behaviour when prefix is unspecified does not change, but its signifier has been changed from empty string "" to "auto"
+
 v22.05 Public major release
  - Various bug fixes.
  - Various optimizations.