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 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 <rkrcmar@redhat.com>
>> ---
>>  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