[Bug c/37990] New: -Os produces redundant instructions

[reza at parvan dot net]

DESCRIPTION:

Simple test case is below.
There is redundant stack manipulation in the assembly output.
Use gcc -Wall -Os -S test.c
-

VERSION INFO:
--
gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --enable-mpfr --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)
---

SOURCE CODE:
--
extern int ii(int i);

void 
mm()
{
int i=99;
more:
i = ii(i);
goto inc;
inc:
if(i==0)
goto more;
i++;
goto more;
}
---


-- 
   Summary: -Os produces redundant instructions
   Product: gcc
   Version: 4.1.2
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: reza at parvan dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37990

[Bug c/37990] -Os produces redundant instructions

[reza at parvan dot net]

--- Comment #2 from reza at parvan dot net  2008-11-02 11:01 ---
Arrrg!  I'm very sorry.  Please ignore this bug report.

I'll change the status to INVALID if that's okay.

Thanks for your response.

Reza.


-- 

reza at parvan dot net changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution||INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37990

[Bug c/45410] New: constant not optimized / propagated

[reza at parvan dot net]

gcc -Wall -O3 -S test.c
doesn't seem to propagate constants correctly.

Notice that ss is declared static, therefor everything is known at compile
time.

Code:
#include 

struct s {int i;int j;};
struct s static ss={77,2};

int
main() 
{
ss.j += 88;
printf("%d\n",sizeof(ss));
return ss.i+ss.j;
};


-- 
   Summary: constant not optimized / propagated
   Product: gcc
   Version: 4.4.3
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
AssignedTo: unassigned at gcc dot gnu dot org
    ReportedBy: reza at parvan dot net
  GCC host triplet: x86 Ubuntu Linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45410

[Bug middle-end/45410] constant not optimized / propagated

[reza at parvan dot net]

--- Comment #5 from reza at parvan dot net  2010-08-27 17:59 ---
(In reply to comment #2)
> That's not the main problem here.  The problem is that this is an aggregate 
> and
> we SRA only automatic vars, not global vars.  And it isn't constant either

Jakub,
Thanks for that insight.  Of course, gcc does a pretty good job (and flow
analysis) if we move 'struct s ss' into function scope.  That seems like a
reasonable workaround for many situations.

Reza.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45410