[Bug target/40603] unnecessary conversion from unsigned byte load to signed byte load

[carrot at google dot com]

------- Comment #3 from carrot at google dot com  2009-07-01 10:24 -------
(In reply to comment #2)
> Subject: Re:   New: unnecessary conversion from unsigned
>         byte load to signed byte load
> 
> 
> > Unfortunately in thumb mode, loading a signed byte costs more than loading an
> > unsigned byte and comparing with 0 has same cost as comparing with 0x7F.
> 
> I don't know of any core where loading a signed byte is more expensive
> than unsigned byte in thumb mode. What did you have in mind ?
> 
> I suspect what you mean is that the sign extension here is not required
> and we could get away with ldrb.
> 
In thumb1, instruction ldrb has an addressing mode of Rn + imm5, but ldrsb has
only addressing mode of Rn + Rm. So loading unsigned byte from p[8] needs only
one instruction
    ldrb r3, [r0, 8]

But loading singed byte from p[8] needs two instructions:

    mov   r3, 8
    ldrsb r3, [r0, r3]

So in this case (base + constant offset), loading a signed byte is more
expensive than unsigned byte in thumb mode.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40603