Re: [Qemu-devel] [PATCH 05/18] target-ppc: Add ISA 2.06 divwe[u][o] Instructions

[Tom Musta <tommusta@gmail.com>]

On 12/9/2013 6:26 PM, Richard Henderson wrote:

>> +        tcg_gen_shli_i64(ra, cpu_gpr[rA(ctx->opcode)], 32);                   \
>> +        /* check for MIN div -1 */                                            \
>> +        int l3 = gen_new_label();                                             \
>> +        tcg_gen_brcondi_i64(TCG_COND_NE, rb, -1l, l3);                        \
>> +        tcg_gen_brcondi_i64(TCG_COND_EQ, ra, INT64_MIN, lbl_ov);              \
> 
> The second test can never be true, since ra has 32 zero bits.
> Thus the first test is also pointless.

Hmm.  Consider the case where GPR[RA] = 0xUUUUUUUU_80000000 (U=don't care) and
GPR[RB] = 0xUUUUUUUU_FFFFFFFF.  Without these checks, I do not believe overflow
will be properly detected.

> 
>> +    gen_set_label(lbl_ov); /* overflow handling */                            \
>> +                                                                              \
>> +    if (signed) {                                                             \
>> +        tcg_gen_sari_i64(cpu_gpr[rD(ctx->opcode)], ra, 63);                   \
>> +    } else {                                                                  \
>> +        tcg_gen_movi_i64(cpu_gpr[rD(ctx->opcode)], 0);                        \
>> +    }                                                                         \
> 
> Divide by zero from the signed case reads an uninitialized ra here.  While it's
> true that the result is undefined, I don't think we want to expose
> uninitialized reads to the TCG optimizer.
> 
> Although... what is that sari for anyway?  The result is undefined in the
> non-div-by-zero overflow case as well.  We might as well use 0, or even
> 0xdeadbeef, all the time, no?

I don't disagree with the comment.  I was being consistent with existing code for
divw/divd.  I suspect whomever wrote this was trying to match the hardware's
implementation.  But 0 is certainly a perfectly good undefined value and would
simplify the code.