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

[Michael Walle]

Am 2015-02-20 15:36, schrieb Paolo Bonzini:
> On 20/02/2015 15:18, Radim Krčmář wrote:
> > 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.
> >
> > Refactored the code a bit to avoid the GCC warning, in an 
> > objectionable
> > way,
> >   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];
> >                 ^
> >
> > Signed-off-by: Radim Krčmář <address@hidden>
> > ---
> >  hw/misc/milkymist-pfpu.c | 18 ++++++++----------
> >  1 file changed, 8 insertions(+), 10 deletions(-)
> >
> > diff --git a/hw/misc/milkymist-pfpu.c b/hw/misc/milkymist-pfpu.c
> > index 609f33f9cd14..133f5b8c5153 100644
> > --- a/hw/misc/milkymist-pfpu.c
> > +++ b/hw/misc/milkymist-pfpu.c
> > @@ -164,6 +164,13 @@ output_queue_advance(MilkymistPFPUState *s)
> >  static int pfpu_decode_insn(MilkymistPFPUState *s)
> >  {
> >      uint32_t pc = s->regs[R_PC];
> > +
> > +    if (pc > MICROCODE_WORDS) {
> > +        error_report("milkymist_pfpu: too many instructions "
> > +                     "executed in microcode. No VECTOUT?");
> > +        return 0;
> > +    }
> > +

I don't like this syntax, eg declaration, statements, declaration. Can 
you just declare the variable first and then assign them? Also the error 
message is then misleading. I'd prefer something like "milkymist_pfpu: 
program counter out of bounds. No VECTOUT?"

> >      uint32_t insn = s->microcode[pc];
> >      uint32_t reg_a = (insn >> 18) & 0x7f;
> >      uint32_t reg_b = (insn >> 11) & 0x7f;
> > @@ -348,7 +355,6 @@ static int pfpu_decode_insn(MilkymistPFPUState *s)
> >  static void pfpu_start(MilkymistPFPUState *s)
> >  {
> >      int x, y;
> > -    int i;
> >
> >      for (y = 0; y <= s->regs[R_VMESHLAST]; y++) {
> >          for (x = 0; x <= s->regs[R_HMESHLAST]; x++) {
> > @@ -359,15 +365,7 @@ static void pfpu_start(MilkymistPFPUState *s)
> >              s->gp_regs[GPR_Y] = y;
> >
> >              /* run microcode on this position */
> > -            i = 0;
> > -            while (pfpu_decode_insn(s)) {
> > -                /* decode at most MICROCODE_WORDS instructions */
> > -                if (i++ >= MICROCODE_WORDS) {
>
> Isn't the fix just to say "++i" instead of "i++"?

In the first run, s->regs[R_PC] may have any value, therefore the "insn 
= s->microcode[pc]" from above may access out of bounds.

-michael