Re: [Qemu-devel] [PATCH 2/2] milkymist-pfpu: fix GCC 5.0.0 aggressive-loop-optimizations warning

[Michael Walle <michael@walle.cc>]

Am 2015-02-20 17:06, schrieb Radim Krčmář:
> man gcc:
>   Warn if in a loop with constant number of iterations the compiler
>   detects undefined behavior in some statement during one or more of
>   the iterations.
> 
> Milkymist pfpu has no jump instructions, so checking for 
> MICROCODE_WORDS
> instructions should have kept us in bounds of s->microcode, but i++
> allowed one loop too many,
> 
>   hw/misc/milkymist-pfpu.c: In function ‘pfpu_write’:
>   hw/misc/milkymist-pfpu.c:365:20: error: loop exit may only be
> reached after undefined behavior
> [-Werror=aggressive-loop-optimizations]
>                    if (i++ >= MICROCODE_WORDS) {
>                       ^
>   hw/misc/milkymist-pfpu.c:167:14: note: possible undefined statement 
> is here
>        uint32_t insn = s->microcode[pc];
>                 ^
> 
> The code can still access out of bounds, because it presumes that PC 
> register
> always begins at 0, and we allow writing to it.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>  v2: used a simpler solution [Paolo, Peter]
> 
>  hw/misc/milkymist-pfpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/misc/milkymist-pfpu.c b/hw/misc/milkymist-pfpu.c
> index 609f33f9cd14..08b604f13f4b 100644
> --- a/hw/misc/milkymist-pfpu.c
> +++ b/hw/misc/milkymist-pfpu.c
> @@ -362,7 +362,7 @@ static void pfpu_start(MilkymistPFPUState *s)
>              i = 0;
>              while (pfpu_decode_insn(s)) {
>                  /* decode at most MICROCODE_WORDS instructions */
> -                if (i++ >= MICROCODE_WORDS) {
> +                if (++i >= MICROCODE_WORDS) {
>                      error_report("milkymist_pfpu: too many 
> instructions "
>                              "executed in microcode. No VECTOUT?");
>                      break;

Acked-by: Michael Walle <michael@walle.cc>