[Bug inline-asm/32971] New: Inline asm with offset addressing fails to compile in 4.0+ without -O

[Bug inline-asm/32971] New: Inline asm with offset addressing fails to compile in 4.0+ without -O

[kinetik at orcon dot net dot nz]

The following test case fails to compile without optimization with gcc 4.3.0
(20070503 (experimental)), 4.1.2 (20070502 (Red Hat 4.1.2-12)), and 4.0.4
(20060904 (prerelease) (Debian 4.0.3-7)), but succeeds with 3.3.6 (Debian
1:3.3.6-15) and 3.4.6 (Debian 3.4.6-5).  With optimization (-O and -O2), it
compiles fine with all of the gcc versions mentioned above.

The failing configurations are generating invalid assembly and gas emits the
following error:
/tmp/ccnbmIy1.s:12: Error: register value used as expression

Looking at the assembly emitted by gcc, the failing case looks like:
        lea (%edx)(%eax), %eax

..and with the working configurations, I get the expected code:
        lea 4(%eax), %eax

#include <stddef.h>

struct coord { int x; int y; };

int *
foo(struct coord * c)
{
  int * i;
  asm ("lea %1(%2), %0"
       : "=r" (i)
       : "m" (*(size_t *) offsetof(struct coord, y)),
         "r" (c)
       : "memory");
  return i;
}


-- 
           Summary: Inline asm with offset addressing fails to compile in
                    4.0+ without -O
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kinetik at orcon dot net dot nz
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32971

[Bug inline-asm/32971] Inline asm with offset addressing fails to compile in 4.0+ without -O

[kinetik at orcon dot net dot nz]

------- Comment #1 from kinetik at orcon dot net dot nz  2007-08-03 04:48 -------
Created an attachment (id=14014)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=14014&action=view)
testcase


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32971

[Bug inline-asm/32971] Inline asm with offset addressing fails to compile in 4.0+ without -O

[kinetik at orcon dot net dot nz]

------- Comment #2 from kinetik at orcon dot net dot nz  2007-08-03 04:51 -------
Using a memory operand to pass the offset value is not ideal in the first
place, but I'm not sure how else I can do it.  It looks like the "i" contraint
(immediate value) is what I want, but the following code:

  asm ("lea %1(%2), %0"
       : "=r" (i)
       : "i" (offsetof(struct coord, y)),
         "r" (c)
       : "memory");

...generates the following assembly, which gas fails to assemble due to syntax
errors:
        lea $4(%eax), %eax


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32971

[Bug inline-asm/32971] Inline asm with offset addressing fails to compile in 4.0+ without -O

[kinetik at flim dot org]

------- Comment #3 from kinetik at flim dot org  2007-08-03 05:11 -------
(In reply to comment #2)
>   asm ("lea %1(%2), %0"
>        : "=r" (i)
>        : "i" (offsetof(struct coord, y)),
>          "r" (c)
>        : "memory");

Bug #29357 suggests that I need to use %c1 rather than %1 here, and indeed this
works and I believe this is the correct solution.

I'm don't know enough to know if the original reason I opened the bug is still
valid (i.e. it's a regression), or if I'm just being bitten by behaviour
changes when I'm abusing the meaning of the operand (I suspect the latter), so
I'll leave the bug open for someone to review and confirm or close.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32971

[Bug inline-asm/32971] Inline asm with offset addressing fails to compile in 4.0+ without -O

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2007-08-03 12:17 -------
%m means it can represent it as memory access which means access it via a
register, this is not a bug.


-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32971