[Bug ada/37604] New: Suboptimal code generation for volatile access

[Bug ada/37604] New: Suboptimal code generation for volatile access

[sam at gcc dot gnu dot org]

The following code generates suboptimal code on i686-pc-linux-gnu:

with Interfaces; use Interfaces;
with System.Storage_Elements;
package A is
   Var : unsigned_8;
   for Var'Address use System.Storage_Elements.To_Address (1000);
   pragma Volatile (Var);
end A;
with A;
with Interfaces; use Interfaces;
procedure U is
begin
   A.Var := A.Var or 1;
   A.Var := A.Var or 2;
end U;

With -O3 -fomit-frame-pointer, we get:

_ada_u:
        movl    a__var, %edx     <=== indirection instead of...
        movzbl  (%edx), %eax     <=== ...direct access here
        orl     $1, %eax
        movb    %al, (%edx)
        movl    a__var, %edx     <=== useless reloading of a__var
        movzbl  (%edx), %eax
        orl     $2, %eax
        movb    %al, (%edx)
        ret

If the variable is declared in the same compilation unit, we get:

_ada_u:
        movzbl  1000, %eax
        orl     $1, %eax
        movb    %al, 1000
        movzbl  1000, %eax
        orl     $2, %eax
        movb    %al, 1000
        ret

We should probably get the same code in both cases.


-- 
           Summary: Suboptimal code generation for volatile access
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: ada
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sam at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37604

[Bug ada/37604] Suboptimal code generation for volatile access

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2008-09-21 12:20 -------
We don't try to optimize volatile accesses.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37604

[Bug ada/37604] Suboptimal code generation for volatile access

[sam at gcc dot gnu dot org]

------- Comment #2 from sam at gcc dot gnu dot org  2008-09-21 12:23 -------
Well, in the second example I give (variable declared in the same compilation
unit), the access is optimized. What I don't get is the difference between both
cases, while the compiler does have the same information at its disposal.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37604