klibc - Mar 2020 - [PATCH v2 1/5] Kbuild: add support for clang builds

From: Michael Davidson <md at google.com>

Add cc-name to klibc/scripts/Kbuild.include.

Make optimization flags not supported by clang conditional on cc-name.

Don't use -nostdinc when building klibc.
  While klibc doesn't (shouldn't) use the standard header files
  supplied by the toolchain, it does still need to be able to find
  the compiler-specific header file <stdarg.h>. With gcc this can
  be achieved by specifying "-iwithprefix include" in addition to
  "-nostdinc". clang behaves differently and the easiest work around
  is to simply not specify "-nostdinc" and rely on the fact that
  the various include paths have been specified correctly so that
  the only header file actually pulled in from the standard location
  is <stdarg.h>.

Disable the use of --print-libgcc when using clang.

Explictly specifying the path to compiler specific libraries
  appears to be unnecessary since klibc never specifies -nostdlib
  when linking binaries, meaning that any such libraries should
  already be available.

Signed-off-by: Bill Wendling <morbo at google.com>
---
 scripts/Kbuild.include        |  4 ++++
 scripts/Kbuild.klibc          | 17 +++++++++++++++--
 usr/klibc/arch/x86_64/MCONFIG | 10 ++++++++--
 3 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index a048ec7a4c89..de31a0c5efa5 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -127,6 +127,10 @@ cc-option-align = $(subst -functions=0,,\
 cc-disable-warning = $(call try-run,\
 	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -W$(strip $(1)) -c -xc /dev/null -o
"$$TMP",-Wno-$(strip $(1)))
 
+# cc-name
+# Expands to either gcc or clang
+cc-name = $(shell $(CC) -v 2>&1 | grep -q "clang version"
&& echo clang || echo gcc)
+
 # cc-version
 # Usage gcc-ver := $(call cc-version)
 cc-version = $(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh $(CC))
diff --git a/scripts/Kbuild.klibc b/scripts/Kbuild.klibc
index afc9a546feca..411d7ea9bea6 100644
--- a/scripts/Kbuild.klibc
+++ b/scripts/Kbuild.klibc
@@ -107,8 +107,15 @@ KLIBCOBJCOPY     := $(OBJCOPY)
 KLIBCOBJDUMP     := $(OBJDUMP)
 
 # klibc include paths
-KLIBCCPPFLAGS    := -nostdinc -iwithprefix include \
-		    -I$(KLIBCINC)/arch/$(KLIBCARCHDIR)	\
+ifeq ($(cc-name),clang)
+# clang can't find the compiler-specific header <stdarg.h> if
-nostdinc
+# is specified so we have to omit it along with the -iwithprefix option.
+# See b/35394554.
+KLIBCCPPFLAGS    :+else
+KLIBCCPPFLAGS    := -nostdinc -iwithprefix include
+endif
+KLIBCCPPFLAGS    += -I$(KLIBCINC)/arch/$(KLIBCARCHDIR)	\
                     -I$(KLIBCINC)/bits$(KLIBCBITSIZE)	\
 		    -I$(KLIBCOBJ)/../include		\
                     -I$(KLIBCINC)
@@ -128,7 +135,13 @@ KLIBCCFLAGS      += $(KLIBCCPPFLAGS) $(KLIBCREQFLAGS)
$(KLIBCARCHREQFLAGS)  \
 KLIBCAFLAGS      += -D__ASSEMBLY__ -Wa,--noexecstack $(KLIBCCFLAGS)
 KLIBCSTRIPFLAGS  += --strip-all -R .comment -R .note
 
+# KLIBCLIBGCC_DEF appears to be unnecessary since klibc never uses -nostdlib
+# when linking binaries, which means that any compiler specific libraries
+# should be available automatically, no need to speciy an explicit path.
+# Don't attempt to set it if we are using clang.
+ifneq ($(cc-name),clang)
 KLIBCLIBGCC_DEF  := $(shell $(KLIBCCC) $(KLIBCCFLAGS) --print-libgcc)
+endif
 KLIBCLIBGCC	 ?= $(KLIBCLIBGCC_DEF)
 KLIBCCRT0        := $(KLIBCOBJ)/arch/$(KLIBCARCHDIR)/crt0.o
 KLIBCLIBC        := $(KLIBCOBJ)/libc.a
diff --git a/usr/klibc/arch/x86_64/MCONFIG b/usr/klibc/arch/x86_64/MCONFIG
index c5f2fa265a26..77e2ad279884 100644
--- a/usr/klibc/arch/x86_64/MCONFIG
+++ b/usr/klibc/arch/x86_64/MCONFIG
@@ -15,13 +15,19 @@
 # debugging using gdb.
 #
 KLIBCARCHREQFLAGS = -m64
-KLIBCOPTFLAGS	+= -Os -fomit-frame-pointer -mno-sse \
-		-falign-functions=1 -falign-jumps=1 -falign-loops=1
+KLIBCOPTFLAGS	+= -Os -fomit-frame-pointer -mno-sse
 ifeq ($(DEBUG),y)
 KLIBCOPTFLAGS     += -g
 else
 KLIBCOPTFLAGS     += -fno-asynchronous-unwind-tables
 endif
+
+ifneq ($(cc-name),clang)
+# These options are not supported by clang but only result in a warning
+# so we can't use a cc-option test to filter them.
+KLIBCOPTFLAGS     += -falign-functions=1 -falign-jumps=1 -falign-loops=1
+endif
+
 KLIBCBITSIZE      = 64
 KLIBCLDFLAGS      = -m elf_x86_64
 
-- 
2.26.0.rc2.310.g2932bb562d-goog

On Fri, 2020-03-27 at 15:29 -0700, Bill Wendling wrote:
> From: Michael Davidson <md at google.com>
> 
> Add cc-name to klibc/scripts/Kbuild.include.
[...]
> --- a/scripts/Kbuild.klibc
> +++ b/scripts/Kbuild.klibc
> @@ -107,8 +107,15 @@ KLIBCOBJCOPY     := $(OBJCOPY)
>  KLIBCOBJDUMP     := $(OBJDUMP)
>  
>  # klibc include paths
> -KLIBCCPPFLAGS    := -nostdinc -iwithprefix include \
> -		    -I$(KLIBCINC)/arch/$(KLIBCARCHDIR)	\
> +ifeq ($(cc-name),clang)
> +# clang can't find the compiler-specific header <stdarg.h> if
-nostdinc
> +# is specified so we have to omit it along with the -iwithprefix option.
> +# See b/35394554.
This seems to be an internal bug number.  It doesn't belong here.
> +KLIBCCPPFLAGS    :> +else
> +KLIBCCPPFLAGS    := -nostdinc -iwithprefix include
> +endif
It might make more sense to include these options only if cc-name is
"gcc".  (Doesn't make any difference now, but if we ever add a
third
compiler then it seems like it ought to work without this.)
> +KLIBCCPPFLAGS    += -I$(KLIBCINC)/arch/$(KLIBCARCHDIR)	\
>                      -I$(KLIBCINC)/bits$(KLIBCBITSIZE)	\
>  		    -I$(KLIBCOBJ)/../include		\
>                      -I$(KLIBCINC)
> @@ -128,7 +135,13 @@ KLIBCCFLAGS      += $(KLIBCCPPFLAGS) $(KLIBCREQFLAGS)
$(KLIBCARCHREQFLAGS)  \
>  KLIBCAFLAGS      += -D__ASSEMBLY__ -Wa,--noexecstack $(KLIBCCFLAGS)
>  KLIBCSTRIPFLAGS  += --strip-all -R .comment -R .note
>  
> +# KLIBCLIBGCC_DEF appears to be unnecessary since klibc never uses
-nostdlib
> +# when linking binaries, which means that any compiler specific libraries
> +# should be available automatically, no need to speciy an explicit path.
> +# Don't attempt to set it if we are using clang.
> +ifneq ($(cc-name),clang)
>  KLIBCLIBGCC_DEF  := $(shell $(KLIBCCC) $(KLIBCCFLAGS) --print-libgcc)
> +endif
Seems like this should also check that cc-name is "gcc", then.
>  KLIBCLIBGCC	 ?= $(KLIBCLIBGCC_DEF)
>  KLIBCCRT0        := $(KLIBCOBJ)/arch/$(KLIBCARCHDIR)/crt0.o
>  KLIBCLIBC        := $(KLIBCOBJ)/libc.a
> diff --git a/usr/klibc/arch/x86_64/MCONFIG b/usr/klibc/arch/x86_64/MCONFIG
> index c5f2fa265a26..77e2ad279884 100644
> --- a/usr/klibc/arch/x86_64/MCONFIG
> +++ b/usr/klibc/arch/x86_64/MCONFIG
> @@ -15,13 +15,19 @@
>  # debugging using gdb.
>  #
>  KLIBCARCHREQFLAGS = -m64
> -KLIBCOPTFLAGS	+= -Os -fomit-frame-pointer -mno-sse \
> -		-falign-functions=1 -falign-jumps=1 -falign-loops=1
> +KLIBCOPTFLAGS	+= -Os -fomit-frame-pointer -mno-sse
>  ifeq ($(DEBUG),y)
>  KLIBCOPTFLAGS     += -g
>  else
>  KLIBCOPTFLAGS     += -fno-asynchronous-unwind-tables
>  endif
> +
> +ifneq ($(cc-name),clang)
> +# These options are not supported by clang but only result in a warning
> +# so we can't use a cc-option test to filter them.
> +KLIBCOPTFLAGS     += -falign-functions=1 -falign-jumps=1 -falign-loops=1
> +endif
Same here.  It would be more readable to write:

ifeq ($(cc-name),gcc)
KLIBCOPTFLAGS += gcc-specific-optimisations
endif
ifeq ($(cc-name),clang)
KLIBCOPTFLAGS += clang-specific-optimisations
endif

And this shouldn't need a comment.

Ben.
>  KLIBCBITSIZE      = 64
>  KLIBCLDFLAGS      = -m elf_x86_64


-- 
Ben Hutchings
This sentence contradicts itself - no actually it doesn't.

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>> +ifeq ($(cc-name),clang)
>> +# clang can't find the compiler-specific header <stdarg.h>
if -nostdinc
>> +# is specified so we have to omit it along with the -iwithprefix
option.
>> +KLIBCCPPFLAGS    :>> +else
>> +KLIBCCPPFLAGS    := -nostdinc -iwithprefix include
>> +endif
Is this even a good idea? The -nostdinc is required so that the normal
system libc includes are not used. Omitting it will cause them to be
detected e.g. by autoconf-like machinery, and subsequent use may cause
trouble and/or failure.

I?d suggest to add -nostdinc together with whatever other options are
needed to the clang case; surely the ?compiler-specific header? path
is discoverable?

bye,
//mirabilos
-- 
Solange man keine schmutzigen Tricks macht, und ich meine *wirklich*
schmutzige Tricks, wie bei einer doppelt verketteten Liste beide
Pointer XORen und in nur einem Word speichern, funktioniert Boehm ganz
hervorragend.		-- Andreas Bogk ?ber boehm-gc in d.a.s.r