diff mbox series

[v3,1/2] sparc/build: Put usage of -fcall-used* flags behind cc-option

Message ID 20241029-sparc-cflags-v3-1-b28745a6bd71@protonmail.com
State New
Headers show
Series sparc/build: Rework CFLAGS for clang compatibility | expand

Commit Message

Koakuma via B4 Relay Oct. 29, 2024, 2:49 p.m. UTC 
From: Koakuma <koachan@protonmail.com>

Place -fcall-used* flags behind cc-option so that clang (which doesn't
support them) can still compile the kernel.

This is a safe change, the reasoning is as follows:

In the (normal) 32-bit ABI, %g5 and %g7 is normally reserved, and in
the 64-bit ABI, %g7 is the reserved one.
Linux turns them into volatile registers by the way of -fcall-used-*,
but on the other hand, omitting the flags shouldn't be harmful;
compilers will now simply refuse to touch them, and any assembly
code that happens to touch them would still work like usual (because
Linux' conventions already treats them as volatile anyway).

Signed-off-by: Koakuma <koachan@protonmail.com>
---
 arch/sparc/Makefile      | 4 ++--
 arch/sparc/vdso/Makefile | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

Comments

Nathan Chancellor Oct. 29, 2024, 10:24 p.m. UTC | #1 
On Tue, Oct 29, 2024 at 09:49:07PM +0700, Koakuma via B4 Relay wrote:
> From: Koakuma <koachan@protonmail.com>
> 
> Place -fcall-used* flags behind cc-option so that clang (which doesn't
> support them) can still compile the kernel.
> 
> This is a safe change, the reasoning is as follows:
> 
> In the (normal) 32-bit ABI, %g5 and %g7 is normally reserved, and in
> the 64-bit ABI, %g7 is the reserved one.
> Linux turns them into volatile registers by the way of -fcall-used-*,
> but on the other hand, omitting the flags shouldn't be harmful;
> compilers will now simply refuse to touch them, and any assembly
> code that happens to touch them would still work like usual (because
> Linux' conventions already treats them as volatile anyway).
> 
> Signed-off-by: Koakuma <koachan@protonmail.com>

Clang builds now succeed with this series and builds with GCC 14.2.0
continue to pass and boot successfully.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

One comment below, please carry these tags forward if there are future
revisions without substantial technical changes.

> ---
>  arch/sparc/Makefile      | 4 ++--
>  arch/sparc/vdso/Makefile | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile
> index 757451c3ea1df63b948e68a45988c78f5974f9ea..0400078076e588be93a702d1c64eb9fd34466075 100644
> --- a/arch/sparc/Makefile
> +++ b/arch/sparc/Makefile
> @@ -29,7 +29,7 @@ UTS_MACHINE    := sparc
>  # versions of gcc.  Some gcc versions won't pass -Av8 to binutils when you
>  # give -mcpu=v8.  This silently worked with older bintutils versions but
>  # does not any more.
> -KBUILD_CFLAGS  += -m32 -mcpu=v8 -pipe -mno-fpu -fcall-used-g5 -fcall-used-g7
> +KBUILD_CFLAGS  += -m32 -mcpu=v8 -pipe -mno-fpu $(call cc-option,-fcall-used-g5) $(call cc-option,-fcall-used-g7)

Small nit, this (and the one in the vdso) could probably be one
cc-option call? Is it likely that one flag would be implemented in the
compiler without the other?

  $(call cc-option,-fcall-used-g5 -fcall-used-g7)

>  KBUILD_CFLAGS  += -Wa,-Av8
>  
>  KBUILD_AFLAGS  += -m32 -Wa,-Av8
> @@ -45,7 +45,7 @@ export BITS   := 64
>  UTS_MACHINE   := sparc64
>  
>  KBUILD_CFLAGS += -m64 -pipe -mno-fpu -mcpu=ultrasparc -mcmodel=medlow
> -KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -fcall-used-g7 -Wno-sign-compare
> +KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 $(call cc-option,-fcall-used-g7) -Wno-sign-compare
>  KBUILD_CFLAGS += -Wa,--undeclared-regs
>  KBUILD_CFLAGS += $(call cc-option,-mtune=ultrasparc3)
>  KBUILD_AFLAGS += -m64 -mcpu=ultrasparc -Wa,--undeclared-regs
> diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile
> index 243dbfc4609d804fc221c3591eebe891107ffdab..50ec2978cda5397841daad6ffdc9682811b9b38e 100644
> --- a/arch/sparc/vdso/Makefile
> +++ b/arch/sparc/vdso/Makefile
> @@ -46,7 +46,7 @@ CFL := $(PROFILING) -mcmodel=medlow -fPIC -O2 -fasynchronous-unwind-tables -m64
>         -fno-omit-frame-pointer -foptimize-sibling-calls \
>         -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
>  
> -SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 -fcall-used-g5 -fcall-used-g7
> +SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 $(call cc-option,-fcall-used-g5) $(call cc-option,-fcall-used-g7)
>  
>  $(vobjs): KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
>  
> 
> -- 
> 2.47.0
> 
> 
>
diff mbox series

Patch

diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile
index 757451c3ea1df63b948e68a45988c78f5974f9ea..0400078076e588be93a702d1c64eb9fd34466075 100644
--- a/arch/sparc/Makefile
+++ b/arch/sparc/Makefile
@@ -29,7 +29,7 @@  UTS_MACHINE    := sparc
 # versions of gcc.  Some gcc versions won't pass -Av8 to binutils when you
 # give -mcpu=v8.  This silently worked with older bintutils versions but
 # does not any more.
-KBUILD_CFLAGS  += -m32 -mcpu=v8 -pipe -mno-fpu -fcall-used-g5 -fcall-used-g7
+KBUILD_CFLAGS  += -m32 -mcpu=v8 -pipe -mno-fpu $(call cc-option,-fcall-used-g5) $(call cc-option,-fcall-used-g7)
 KBUILD_CFLAGS  += -Wa,-Av8
 
 KBUILD_AFLAGS  += -m32 -Wa,-Av8
@@ -45,7 +45,7 @@  export BITS   := 64
 UTS_MACHINE   := sparc64
 
 KBUILD_CFLAGS += -m64 -pipe -mno-fpu -mcpu=ultrasparc -mcmodel=medlow
-KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -fcall-used-g7 -Wno-sign-compare
+KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 $(call cc-option,-fcall-used-g7) -Wno-sign-compare
 KBUILD_CFLAGS += -Wa,--undeclared-regs
 KBUILD_CFLAGS += $(call cc-option,-mtune=ultrasparc3)
 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc -Wa,--undeclared-regs
diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile
index 243dbfc4609d804fc221c3591eebe891107ffdab..50ec2978cda5397841daad6ffdc9682811b9b38e 100644
--- a/arch/sparc/vdso/Makefile
+++ b/arch/sparc/vdso/Makefile
@@ -46,7 +46,7 @@  CFL := $(PROFILING) -mcmodel=medlow -fPIC -O2 -fasynchronous-unwind-tables -m64
        -fno-omit-frame-pointer -foptimize-sibling-calls \
        -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
 
-SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 -fcall-used-g5 -fcall-used-g7
+SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 $(call cc-option,-fcall-used-g5) $(call cc-option,-fcall-used-g7)
 
 $(vobjs): KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)