[PATCH] [powerpc] Tighten contraints for asm constant parameters

[PATCH] [powerpc] Tighten contraints for asm constant parameters

[Paul A. Clarke]

There are a few places where only constants are acceptable for `asm`
parameters, yet the constraint "i" is used.  "i" is for "any integer"
including variables.

Use "n" instead of "i" where constant integers are required.

Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>
---
 sysdeps/powerpc/fpu/fenv_libc.h  | 8 ++++----
 sysdeps/powerpc/test-get_hwcap.c | 8 ++++----
 sysdeps/powerpc/tst-tlsifunc.c   | 2 +-
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/sysdeps/powerpc/fpu/fenv_libc.h b/sysdeps/powerpc/fpu/fenv_libc.h
index dc35b9dbe0d0..a04fb928cae2 100644
--- a/sysdeps/powerpc/fpu/fenv_libc.h
+++ b/sysdeps/powerpc/fpu/fenv_libc.h
@@ -73,7 +73,7 @@ extern const fenv_t *__fe_mask_env (void) attribute_hidden;
     if (__builtin_constant_p (rn))					\
       __asm__ __volatile__ (						\
         ".machine push; .machine \"power9\"; mffscrni %0,%1; .machine pop" \
-        : "=f" (__fr.fenv) : "i" (rn));					\
+        : "=f" (__fr.fenv) : "n" (rn));					\
     else								\
     {									\
       __fr.l = (rn);							\
@@ -135,8 +135,8 @@ extern const fenv_t *__fe_mask_env (void) attribute_hidden;
 /* Set/clear a particular FPSCR bit (for instance,
    reset_fpscr_bit(FPSCR_VE);
    prevents INVALID exceptions from being raised).  */
-#define set_fpscr_bit(x) asm volatile ("mtfsb1 %0" : : "i"(x))
-#define reset_fpscr_bit(x) asm volatile ("mtfsb0 %0" : : "i"(x))
+#define set_fpscr_bit(x) asm volatile ("mtfsb1 %0" : : "n"(x))
+#define reset_fpscr_bit(x) asm volatile ("mtfsb0 %0" : : "n"(x))
 
 typedef union
 {
@@ -184,7 +184,7 @@ __fesetround_inline_nocheck (const int round)
   if (__glibc_likely (GLRO(dl_hwcap2) & PPC_FEATURE2_ARCH_3_00))
     __fe_mffscrn (round);
   else
-    asm volatile ("mtfsfi 7,%0" : : "i" (round));
+    asm volatile ("mtfsfi 7,%0" : : "n" (round));
 #endif
 }
 
diff --git a/sysdeps/powerpc/test-get_hwcap.c b/sysdeps/powerpc/test-get_hwcap.c
index b5cef93cddd4..a64b63080756 100644
--- a/sysdeps/powerpc/test-get_hwcap.c
+++ b/sysdeps/powerpc/test-get_hwcap.c
@@ -63,16 +63,16 @@ uint64_t check_tcbhwcap (long tid)
 #ifdef __powerpc64__
   __asm__  ("ld %0,%1(%2)\n"
 	    : "=r" (tcb_hwcap)
-	    : "i" (__HWCAPOFF), "b" (__tp));
+	    : "n" (__HWCAPOFF), "b" (__tp));
 #else
   uint64_t h1, h2;
 
   __asm__ ("lwz %0,%1(%2)\n"
       : "=r" (h1)
-      : "i" (__HWCAPOFF), "b" (__tp));
+      : "n" (__HWCAPOFF), "b" (__tp));
   __asm__ ("lwz %0,%1(%2)\n"
       : "=r" (h2)
-      : "i" (__HWCAP2OFF), "b" (__tp));
+      : "n" (__HWCAP2OFF), "b" (__tp));
   tcb_hwcap = (h1 >> 32) << 32 | (h2 >> 32);
 #endif
 
@@ -117,7 +117,7 @@ uint64_t check_tcbhwcap (long tid)
   /* Same test for the platform number.  */
   __asm__  ("lwz %0,%1(%2)\n"
 	    : "=r" (tcb_at_platform)
-	    : "i" (__ATPLATOFF), "b" (__tp));
+	    : "n" (__ATPLATOFF), "b" (__tp));
 
   at_platform_string = (const char *) getauxval (AT_PLATFORM);
   at_platform = _dl_string_platform (at_platform_string);
diff --git a/sysdeps/powerpc/tst-tlsifunc.c b/sysdeps/powerpc/tst-tlsifunc.c
index c8c0bada4547..f2eaf11bb407 100644
--- a/sysdeps/powerpc/tst-tlsifunc.c
+++ b/sysdeps/powerpc/tst-tlsifunc.c
@@ -49,7 +49,7 @@ get_platform (void)
 
   __asm__  ("lwz %0,%1(%2)\n"
 	    : "=r" (tmp)
-	    : "i" (__ATPLATOFF), "b" (tp));
+	    : "n" (__ATPLATOFF), "b" (tp));
 
   return tmp;
 }
-- 
2.27.0

Re: [PATCH] [powerpc] Tighten contraints for asm constant parameters

[Florian Weimer]

* Paul A. Clarke via Libc-alpha:

> There are a few places where only constants are acceptable for `asm`
> parameters, yet the constraint "i" is used.  "i" is for "any integer"
> including variables.
>
> Use "n" instead of "i" where constant integers are required.

This doesn't seem to match the GCC documentation:

| 'i'
|      An immediate integer operand (one with constant value) is allowed.
|      This includes symbolic constants whose values will be known only at
|      assembly time or later.

And the RS6000 documentation does not override that.

Thanks,
Florian

Re: [PATCH] [powerpc] Tighten contraints for asm constant parameters

[Paul A. Clarke]

On Tue, Oct 19, 2021 at 05:26:36PM +0200, Florian Weimer wrote:
> * Paul A. Clarke via Libc-alpha:
> 
> > There are a few places where only constants are acceptable for `asm`
> > parameters, yet the constraint "i" is used.  "i" is for "any integer"
> > including variables.
> >
> > Use "n" instead of "i" where constant integers are required.
> 
> This doesn't seem to match the GCC documentation:
> 
> | 'i'
> |      An immediate integer operand (one with constant value) is allowed.
> |      This includes symbolic constants whose values will be known only at
> |      assembly time or later.
> 
> And the RS6000 documentation does not override that.

I was too loose with my characterization of 'i', in contrast to 'n':

| ‘n’
|      An immediate integer operand with a known numeric value is allowed.
|      Many systems cannot support assembly-time constants for operands less
|      than a word wide. Constraints for these operands should use ‘n’
|      rather than ‘i’.

The cases changed by the patch require a *known numeric value*, as they are
used as immediate values (hardcoded in the generated opcode).

I will reword to:
  There are a few places where only known numeric values are acceptable for
  `asm` parameters, yet the constraint "i" is used.  "i" can include
  "symbolic constants whose values will be known only at assembly time or
  later."

  Use "n" instead of "i" where known numeric values are required.

Does that work better?

PC

Re: [PATCH] [powerpc] Tighten contraints for asm constant parameters

[Florian Weimer]

* Paul A. Clarke:

> I was too loose with my characterization of 'i', in contrast to 'n':
>
> | ‘n’
> |      An immediate integer operand with a known numeric value is allowed.
> |      Many systems cannot support assembly-time constants for operands less
> |      than a word wide. Constraints for these operands should use ‘n’
> |      rather than ‘i’.
>
> The cases changed by the patch require a *known numeric value*, as they are
> used as immediate values (hardcoded in the generated opcode).
>
> I will reword to:
>   There are a few places where only known numeric values are acceptable for
>   `asm` parameters, yet the constraint "i" is used.  "i" can include
>   "symbolic constants whose values will be known only at assembly time or
>   later."
>
>   Use "n" instead of "i" where known numeric values are required.
>
> Does that work better?

Yes, that explains the difference.  Thanks!

Florian

Re: [PATCH] [powerpc] Tighten contraints for asm constant parameters

[Tulio Magno Quites Machado Filho]

"Paul A. Clarke" <pc@us.ibm.com> writes:

> There are a few places where only constants are acceptable for `asm`
> parameters, yet the constraint "i" is used.  "i" is for "any integer"
> including variables.
>
> Use "n" instead of "i" where constant integers are required.
>
> Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>

LGTM.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>

-- 
Tulio Magno