arch/x86/boot/main.c | 4 ++-- arch/x86/include/asm/alternative.h | 22 +++++++++++----------- arch/x86/include/asm/apic.h | 2 +- arch/x86/include/asm/atomic64_32.h | 2 +- arch/x86/include/asm/cpufeature.h | 4 ++-- arch/x86/include/asm/irq_stack.h | 2 +- arch/x86/include/asm/processor.h | 6 +++--- arch/x86/include/asm/special_insns.h | 4 ++-- arch/x86/include/asm/uaccess.h | 4 ++-- 9 files changed, 25 insertions(+), 25 deletions(-)
The "P" asm operand modifier is a x86 target-specific modifier.
For x86_64, when used with a symbol reference, the "P" modifier
emits "sym" instead of "sym(%rip)". When used with a constant, the
"P" modifier emits "cst" instead of "$cst". This property is used to
emit bare symbol references and bare constants without all
syntax-specific prefixes.
The generic "c", "n" and "a" operand modifiers should be used instead.
The following table shows the modifiers supported by all targets and
their effects:
Modifier Description
-----------------------------------------------------------
'c' Require a constant operand and print the
constant expression with no punctuation.
'n' Like '%c' except that the value of the constant
is negated before printing.
'a' Substitute a memory reference, with the actual
operand treated as the address. This may be
useful when outputting a "load address"
instruction, because often the assembler syntax
for such an instruction requires you to write
the operand as if it were a memory reference.
Also note that unlike GCC, clang emits %rip-relative symbol
reference with "P" asm operand modifier, so the patch also unifies
symbol handling with both compilers.
No functional changes intended.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Uros Bizjak (3):
x86/asm: Remove %P operand modifier from altinstr asm templates
x86/asm: Use %c/%n instead of %P operand modifier in asm templates
x86/asm: Use %a instead of %P operand modifier in asm templates
arch/x86/boot/main.c | 4 ++--
arch/x86/include/asm/alternative.h | 22 +++++++++++-----------
arch/x86/include/asm/apic.h | 2 +-
arch/x86/include/asm/atomic64_32.h | 2 +-
arch/x86/include/asm/cpufeature.h | 4 ++--
arch/x86/include/asm/irq_stack.h | 2 +-
arch/x86/include/asm/processor.h | 6 +++---
arch/x86/include/asm/special_insns.h | 4 ++--
arch/x86/include/asm/uaccess.h | 4 ++--
9 files changed, 25 insertions(+), 25 deletions(-)
--
2.44.0
On Tue, Mar 19, 2024 at 6:44 AM Uros Bizjak <ubizjak@gmail.com> wrote: > > The "P" asm operand modifier is a x86 target-specific modifier. > > For x86_64, when used with a symbol reference, the "P" modifier > emits "sym" instead of "sym(%rip)". When used with a constant, the > "P" modifier emits "cst" instead of "$cst". This property is used to > emit bare symbol references and bare constants without all > syntax-specific prefixes. > > The generic "c", "n" and "a" operand modifiers should be used instead. > The following table shows the modifiers supported by all targets and > their effects: > > Modifier Description > ----------------------------------------------------------- > 'c' Require a constant operand and print the > constant expression with no punctuation. > 'n' Like '%c' except that the value of the constant > is negated before printing. > 'a' Substitute a memory reference, with the actual > operand treated as the address. This may be > useful when outputting a "load address" > instruction, because often the assembler syntax > for such an instruction requires you to write > the operand as if it were a memory reference. > > Also note that unlike GCC, clang emits %rip-relative symbol > reference with "P" asm operand modifier, so the patch also unifies > symbol handling with both compilers. > > No functional changes intended. > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > Cc: Andy Lutomirski <luto@kernel.org> > Cc: Brian Gerst <brgerst@gmail.com> > Cc: Denys Vlasenko <dvlasenk@redhat.com> > Cc: H. Peter Anvin <hpa@zytor.com> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Josh Poimboeuf <jpoimboe@redhat.com> > Cc: Sean Christopherson <seanjc@google.com> > > Uros Bizjak (3): > x86/asm: Remove %P operand modifier from altinstr asm templates > x86/asm: Use %c/%n instead of %P operand modifier in asm templates > x86/asm: Use %a instead of %P operand modifier in asm templates > > arch/x86/boot/main.c | 4 ++-- > arch/x86/include/asm/alternative.h | 22 +++++++++++----------- > arch/x86/include/asm/apic.h | 2 +- > arch/x86/include/asm/atomic64_32.h | 2 +- > arch/x86/include/asm/cpufeature.h | 4 ++-- > arch/x86/include/asm/irq_stack.h | 2 +- > arch/x86/include/asm/processor.h | 6 +++--- > arch/x86/include/asm/special_insns.h | 4 ++-- > arch/x86/include/asm/uaccess.h | 4 ++-- > 9 files changed, 25 insertions(+), 25 deletions(-) > > -- > 2.44.0 > My one concern is does this work with older compilers? Technically, GCC 5.1 is still supported. I seem to recall some issues around these modifiers in the past, but don't remember any details. Otherwise, it's a nice cleanup. Brian Gerst
On Tue, Mar 19, 2024 at 2:10 PM Brian Gerst <brgerst@gmail.com> wrote: > > On Tue, Mar 19, 2024 at 6:44 AM Uros Bizjak <ubizjak@gmail.com> wrote: > > > > The "P" asm operand modifier is a x86 target-specific modifier. > > > > For x86_64, when used with a symbol reference, the "P" modifier > > emits "sym" instead of "sym(%rip)". When used with a constant, the > > "P" modifier emits "cst" instead of "$cst". This property is used to > > emit bare symbol references and bare constants without all > > syntax-specific prefixes. > > > > The generic "c", "n" and "a" operand modifiers should be used instead. > > The following table shows the modifiers supported by all targets and > > their effects: > > > > Modifier Description > > ----------------------------------------------------------- > > 'c' Require a constant operand and print the > > constant expression with no punctuation. > > 'n' Like '%c' except that the value of the constant > > is negated before printing. > > 'a' Substitute a memory reference, with the actual > > operand treated as the address. This may be > > useful when outputting a "load address" > > instruction, because often the assembler syntax > > for such an instruction requires you to write > > the operand as if it were a memory reference. > > > > Also note that unlike GCC, clang emits %rip-relative symbol > > reference with "P" asm operand modifier, so the patch also unifies > > symbol handling with both compilers. > > > > No functional changes intended. > > > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > > Cc: Andy Lutomirski <luto@kernel.org> > > Cc: Brian Gerst <brgerst@gmail.com> > > Cc: Denys Vlasenko <dvlasenk@redhat.com> > > Cc: H. Peter Anvin <hpa@zytor.com> > > Cc: Linus Torvalds <torvalds@linux-foundation.org> > > Cc: Ingo Molnar <mingo@kernel.org> > > Cc: Josh Poimboeuf <jpoimboe@redhat.com> > > Cc: Sean Christopherson <seanjc@google.com> > > > > Uros Bizjak (3): > > x86/asm: Remove %P operand modifier from altinstr asm templates > > x86/asm: Use %c/%n instead of %P operand modifier in asm templates > > x86/asm: Use %a instead of %P operand modifier in asm templates > > > > arch/x86/boot/main.c | 4 ++-- > > arch/x86/include/asm/alternative.h | 22 +++++++++++----------- > > arch/x86/include/asm/apic.h | 2 +- > > arch/x86/include/asm/atomic64_32.h | 2 +- > > arch/x86/include/asm/cpufeature.h | 4 ++-- > > arch/x86/include/asm/irq_stack.h | 2 +- > > arch/x86/include/asm/processor.h | 6 +++--- > > arch/x86/include/asm/special_insns.h | 4 ++-- > > arch/x86/include/asm/uaccess.h | 4 ++-- > > 9 files changed, 25 insertions(+), 25 deletions(-) > > > > -- > > 2.44.0 > > > > My one concern is does this work with older compilers? Technically, > GCC 5.1 is still supported. I seem to recall some issues around these > modifiers in the past, but don't remember any details. Otherwise, > it's a nice cleanup. It will work. Please see [1] for a testcase. [1] https://godbolt.org/z/eb6rcz8YW Uros.
On Tue, Mar 19, 2024 at 11:40:11AM +0100, Uros Bizjak wrote:
> The "P" asm operand modifier is a x86 target-specific modifier.
>
> For x86_64, when used with a symbol reference, the "P" modifier
> emits "sym" instead of "sym(%rip)". When used with a constant, the
> "P" modifier emits "cst" instead of "$cst". This property is used to
> emit bare symbol references and bare constants without all
> syntax-specific prefixes.
>
> The generic "c", "n" and "a" operand modifiers should be used instead.
> The following table shows the modifiers supported by all targets and
> their effects:
>
> Modifier Description
> -----------------------------------------------------------
> 'c' Require a constant operand and print the
> constant expression with no punctuation.
> 'n' Like '%c' except that the value of the constant
> is negated before printing.
> 'a' Substitute a memory reference, with the actual
> operand treated as the address. This may be
> useful when outputting a "load address"
> instruction, because often the assembler syntax
> for such an instruction requires you to write
> the operand as if it were a memory reference.
>
> Also note that unlike GCC, clang emits %rip-relative symbol
> reference with "P" asm operand modifier, so the patch also unifies
> symbol handling with both compilers.
FTR, I really appreciate the clear explanations of the operand modifiers
along with an example in the commit messages. What they really do and
what they mean have caused some serious head-scratching in the past, up
to the point where I went through gcc sources with Matz' help to figure
out what some of them do.
So thanks!
:-)
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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