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Subject: Re: [Qemu-devel] [PATCH 05/18] target-ppc: Add ISA 2.06 divwe[u][o]
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To: Tom Musta <tommusta@gmail.com>, qemu-devel@nongnu.org
Cc: qemu-ppc@nongnu.org

On 12/09/2013 07:47 AM, Tom Musta wrote:
> +        /* does the result fit in 32 bits? */                                 \
> +        tcg_gen_brcondi_i64(TCG_COND_LT, cpu_gpr[rD(ctx->opcode)], INT32_MIN, \
> +                            lbl_ov);                                          \
> +        tcg_gen_brcondi_i64(TCG_COND_GT, cpu_gpr[rD(ctx->opcode)], INT32_MAX, \
> +                            lbl_ov);                                          \

Better with an extend and single brcond.

> +        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.

> +    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?


r~