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Re: [Qemu-devel] [PATCH v2 13/27] target/arm: convert conversion helpers

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH v2 13/27] target/arm: convert conversion helpers to fpst/ahp_flag
Date: Mon, 14 May 2018 14:41:50 +0100

On 12 May 2018 at 01:42, Richard Henderson <address@hidden> wrote:
> From: Alex Bennée <address@hidden>
> Instead of passing env and leaving it up to the helper to get the
> right fpstatus we pass it explicitly. There was already a get_fpstatus
> helper for neon for the 32 bit code. We also add an get_ahp_flag() for
> passing the state of the alternative FP16 format flag. This leaves
> scope for later tracking the AHP state in translation flags.
> Signed-off-by: Alex Bennée <address@hidden>
> Signed-off-by: Richard Henderson <address@hidden>

> diff --git a/target/arm/translate-a64.c b/target/arm/translate-a64.c
> index a0b0c43d12..570791e7b5 100644
> --- a/target/arm/translate-a64.c
> +++ b/target/arm/translate-a64.c
> @@ -5147,10 +5147,15 @@ static void handle_fp_fcvt(DisasContext *s, int 
> opcode,
>          } else {
>              /* Single to half */
>              TCGv_i32 tcg_rd = tcg_temp_new_i32();
> -            gen_helper_vfp_fcvt_f32_to_f16(tcg_rd, tcg_rn, cpu_env);
> +            TCGv_i32 ahp = get_ahp_flag();
> +            TCGv_ptr fpst = get_fpstatus_ptr(true);

This should be passing in 'false' to get_fpstatus_ptr(), as should
all the other uses where we get an fpst to pass to an f*_to_f16 or
f16_to_f* conversion helper. This is because conversions between
f16 and other sizes ignore FPCR.FZ16 for the 16-bit end, but they
still honour FPCR.FZ for the other end. So you need to pass in the
fpstatus pointer which has the flush-denormals status flags set
to match FPCR.FZ, not the fp16-specific one.

Otherwise you'll generate the wrong exception flags for cases like
 FPCR.FZ = 1; FPCR.FZ16 = 0; s0 = 0x00000001; fcvt h1, s0
(which should give h1 = 0x0000 and set InputDenormal).

-- PMM

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