[Bug target/89502] Inefficient access to stack_guard in TCB

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89502

--- Comment #7 from H.J. Lu  ---
rtx
memory_address_addr_space (machine_mode mode, rtx x, addr_space_t as)
{
  rtx oldx = x; 
  scalar_int_mode address_mode = targetm.addr_space.address_mode (as);

  x = convert_memory_address_addr_space (address_mode, x, as); 

  /* By passing constant addresses through registers
 we get a chance to cse them.  */
  if (! cse_not_expected && CONSTANT_P (x) && CONSTANT_ADDRESS_P (x)) 
x = force_reg (address_mode, x);

cprop undoes it, which isn't enabled by O1.  For x86, using register
instead of a 32-bit constant usually is a performance loss.  Shouldn't
backend have a choice here?

[Bug target/89502] Inefficient access to stack_guard in TCB

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89502

--- Comment #6 from Uroš Bizjak  ---
(In reply to H.J. Lu from comment #5)

> I think we should avoid %fs:(%edx) address.  Is that possible to generate

It is what middle-end expands to, so nothing much that we can do here.

[Bug target/89502] Inefficient access to stack_guard in TCB

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89502

--- Comment #5 from H.J. Lu  ---
(In reply to Uroš Bizjak from comment #3)
> (In reply to H.J. Lu from comment #0)
> > x.s: Assembler messages:
> > x.s:11: Error: can't encode segment `%fs' with 32-bit address
> > x.s:18: Error: can't encode segment `%fs' with 32-bit address
> 
> Is this some new warning, since

Yes. it will be in binutils 2.33.

> GNU assembler version 2.31.1-17.fc29
> 
> happily assembles 
> 
>   movl%fs:(%edx), %ecx
> 
> to:
> 
>8:   64 67 8b 0a mov%fs:(%edx),%ecx
> 
> Anyway, the following patch should fix this issue:
> 
> --cut here--
> diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
> index e77653d66a4..67872e1c15d 100644
> --- a/gcc/config/i386/i386.c
> +++ b/gcc/config/i386/i386.c
> @@ -18542,6 +18542,10 @@ ix86_print_operand_address_as (FILE *file, rtx addr,
>  
>if (!ADDR_SPACE_GENERIC_P (as))
>  {
> +  /* We can't have 0x67 prefix with segment override.  */
> +  if (TARGET_64BIT)
> +   code = 'q';
> +
>if (ASSEMBLER_DIALECT == ASM_ATT)
> putc ('%', file);
>  
> --cut here--

I think we should avoid %fs:(%edx) address.  Is that possible to generate
%fs:24 even at -O0?  Glibc can do it at -O0:

[hjl@gnu-cfl-1 gcc]$ cat x.i
int
foo (void)
{
  int i;
  asm volatile ("movl %%fs:%P1, %0" : "=r" (i) : "i" (24)); 
  return i;
}
[hjl@gnu-cfl-1 gcc]$ gcc -S -O0 x.i 
[hjl@gnu-cfl-1 gcc]$ cat x.s
.file   "x.i"
.text
.globl  foo
.type   foo, @function
foo:
.LFB0:
.cfi_startproc
pushq   %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq%rsp, %rbp
.cfi_def_cfa_register 6
#APP
# 5 "x.i" 1
movl %fs:24, %eax
# 0 "" 2
#NO_APP
movl%eax, -4(%rbp)
movl-4(%rbp), %eax
popq%rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size   foo, .-foo
.ident  "GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
.section.note.GNU-stack,"",@progbits
[hjl@gnu-cfl-1 gcc]$

[Bug target/89502] Inefficient access to stack_guard in TCB

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89502

--- Comment #4 from Uroš Bizjak  ---
(In reply to H.J. Lu from comment #1)
> Ever better, we can use UNSPEC_TP to handle it:
> 
>   movl%fs:24, %ecx
> 
> to save a register.

We do, with -O2, but -O1 CSEs the constant into the register for some reason.

[Bug target/89502] Inefficient access to stack_guard in TCB

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89502

--- Comment #3 from Uroš Bizjak  ---
(In reply to H.J. Lu from comment #0)
> x.s: Assembler messages:
> x.s:11: Error: can't encode segment `%fs' with 32-bit address
> x.s:18: Error: can't encode segment `%fs' with 32-bit address

Is this some new warning, since

GNU assembler version 2.31.1-17.fc29

happily assembles 

movl%fs:(%edx), %ecx

to:

   8:   64 67 8b 0a mov%fs:(%edx),%ecx

Anyway, the following patch should fix this issue:

--cut here--
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index e77653d66a4..67872e1c15d 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -18542,6 +18542,10 @@ ix86_print_operand_address_as (FILE *file, rtx addr,

   if (!ADDR_SPACE_GENERIC_P (as))
 {
+  /* We can't have 0x67 prefix with segment override.  */
+  if (TARGET_64BIT)
+   code = 'q';
+
   if (ASSEMBLER_DIALECT == ASM_ATT)
putc ('%', file);

--cut here--