[Bug inline-asm/11203] source doesn't compile with -O0 but they compile with -O3

["xyzzy at speakeasy dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #36 from xyzzy at speakeasy dot org  2006-11-08 20:03 -------
(In reply to comment #21)
>     asm volatile(""
>         : "=m" (*(unsigned int*)(src + 0*stride)),
>           "=m" (*(unsigned int*)(src + 1*stride)),
>           "=m" (*(unsigned int*)(src + 2*stride)),
>           "=m" (*(unsigned int*)(src + 3*stride)),
>           "=m" (*(unsigned int*)(src + 4*stride)),
>           "=m" (*(unsigned int*)(src + 5*stride)),
>           "=m" (*(unsigned int*)(src + 6*stride)),
>           "=m" (*(unsigned int*)(src + 7*stride))
>     );

(In reply to comment #26)
> it might also happen that in some intentionally overconstrained cases it ends up
> searching the whole 5040 possible assignments of 7 registers onto 7 non memory
> operands but still it wont fail

The example Martin gave has *8* operands.  You can try every possible direct
mapping of those 8 addresses to just 7 registers, but they will obviously all
fail.  Except with ia32 addressing modes it _can_ be done, and with only 4
registers.

reg1 = src, reg2 = stride, reg3 = src+stride*3, reg4 = src+stride*6
Then the 8 memory operands are:
(reg1), (reg1,reg2,1), (reg1,reg2,2), (reg3),
(reg1,reg2,4), (reg3,reg2,2), (reg4), (reg3,reg2,4)

When one considers all the addressing modes, there are not just 7 possible
registers, but (I think) 261 possible addresses.  There are not just 5040
possibilities as Michael said, but over 76 x 10^15 possible ways of assigning
these addresses to 7 operands!  Then each register can be loaded not just with
an address but with some sub-expression too, like how I loaded reg2 with
stride.

Even for ia32, which makes up for its limited number of registers with complex
addressing modes, finding a register allocation that satisfies an asm statement
is not something that can always be done in reasonable time.  If the number of
operands <= number of available registers it should be able to (but gcc
doesn't) always find an allocation (_an_ allocation, not the best allocation).


-- 

xyzzy at speakeasy dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xyzzy at speakeasy dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203