Re: [Qemu-devel] [PATCH v5 22/28] fpu/softfloat: Specialize on snan_bit_is_one

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <richard.henderson@linaro.org> writes:

> Only MIPS requires snan_bit_is_one to be variable.  While we are
> specializing softfloat behaviour, allow other targets to eliminate
> this runtime check.
>
> Cc: Aurelien Jarno <aurelien@aurel32.net>
> Cc: Yongbok Kim <yongbok.kim@mips.com>
> Cc: David Gibson <david@gibson.dropbear.id.au>
> Cc: Alexander Graf <agraf@suse.de>
> Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

>
> ---
> v5
>   - do not remove the set_snan_bit_is_one function
>   - tidy the language in the snan_bit_is_one block comment
> ---
>  fpu/softfloat-specialize.h    | 68 ++++++++++++++++++++++-------------
>  include/fpu/softfloat-types.h |  1 +
>  target/hppa/cpu.c             |  1 -
>  target/ppc/fpu_helper.c       |  1 -
>  target/sh4/cpu.c              |  1 -
>  target/unicore32/cpu.c        |  2 --
>  6 files changed, 44 insertions(+), 30 deletions(-)
>
> diff --git a/fpu/softfloat-specialize.h b/fpu/softfloat-specialize.h
> index d7033b7757..d1e06da75b 100644
> --- a/fpu/softfloat-specialize.h
> +++ b/fpu/softfloat-specialize.h
> @@ -79,13 +79,31 @@ this code that are retained.
>   * version 2 or later. See the COPYING file in the top-level directory.
>   */
>
> -#if defined(TARGET_XTENSA)
>  /* Define for architectures which deviate from IEEE in not supporting
>   * signaling NaNs (so all NaNs are treated as quiet).
>   */
> +#if defined(TARGET_XTENSA)
>  #define NO_SIGNALING_NANS 1
>  #endif
>
> +/* Define how the architecture discriminates signaling NaNs.
> + * This done with the most significant bit of the fraction.
> + * In IEEE 754-1985 this was implementation defined, but in IEEE 754-2008
> + * the msb must be zero.  MIPS is (so far) unique in supporting both the
> + * 2008 revision and backward compatibility with their original choice.
> + * Thus for MIPS we must make the choice at runtime.
> + */
> +static inline flag snan_bit_is_one(float_status *status)
> +{
> +#if defined(TARGET_MIPS)
> +    return status->snan_bit_is_one;
> +#elif defined(TARGET_HPPA) || defined(TARGET_UNICORE32) || defined(TARGET_SH4)
> +    return 1;
> +#else
> +    return 0;
> +#endif
> +}
> +
>  /*----------------------------------------------------------------------------
>  | For the deconstructed floating-point with fraction FRAC, return true
>  | if the fraction represents a signalling NaN; otherwise false.
> @@ -97,7 +115,7 @@ static bool parts_is_snan_frac(uint64_t frac, float_status *status)
>      return false;
>  #else
>      flag msb = extract64(frac, DECOMPOSED_BINARY_POINT - 1, 1);
> -    return msb == status->snan_bit_is_one;
> +    return msb == snan_bit_is_one(status);
>  #endif
>  }
>
> @@ -118,7 +136,7 @@ static FloatParts parts_default_nan(float_status *status)
>  #elif defined(TARGET_HPPA)
>      frac = 1ULL << (DECOMPOSED_BINARY_POINT - 2);
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          frac = (1ULL << (DECOMPOSED_BINARY_POINT - 1)) - 1;
>      } else {
>  #if defined(TARGET_MIPS)
> @@ -151,7 +169,7 @@ static FloatParts parts_silence_nan(FloatParts a, float_status *status)
>      a.frac &= ~(1ULL << (DECOMPOSED_BINARY_POINT - 1));
>      a.frac |= 1ULL << (DECOMPOSED_BINARY_POINT - 2);
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return parts_default_nan(status);
>      } else {
>          a.frac |= 1ULL << (DECOMPOSED_BINARY_POINT - 1);
> @@ -169,7 +187,7 @@ float16 float16_default_nan(float_status *status)
>  #if defined(TARGET_ARM)
>      return const_float16(0x7E00);
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return const_float16(0x7DFF);
>      } else {
>  #if defined(TARGET_MIPS)
> @@ -195,7 +213,7 @@ float32 float32_default_nan(float_status *status)
>  #elif defined(TARGET_HPPA)
>      return const_float32(0x7FA00000);
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return const_float32(0x7FBFFFFF);
>      } else {
>  #if defined(TARGET_MIPS)
> @@ -220,7 +238,7 @@ float64 float64_default_nan(float_status *status)
>  #elif defined(TARGET_HPPA)
>      return const_float64(LIT64(0x7FF4000000000000));
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return const_float64(LIT64(0x7FF7FFFFFFFFFFFF));
>      } else {
>  #if defined(TARGET_MIPS)
> @@ -242,7 +260,7 @@ floatx80 floatx80_default_nan(float_status *status)
>      r.low = LIT64(0xFFFFFFFFFFFFFFFF);
>      r.high = 0x7FFF;
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          r.low = LIT64(0xBFFFFFFFFFFFFFFF);
>          r.high = 0x7FFF;
>      } else {
> @@ -274,7 +292,7 @@ float128 float128_default_nan(float_status *status)
>  {
>      float128 r;
>
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          r.low = LIT64(0xFFFFFFFFFFFFFFFF);
>          r.high = LIT64(0x7FFF7FFFFFFFFFFF);
>      } else {
> @@ -319,7 +337,7 @@ int float16_is_quiet_nan(float16 a_, float_status *status)
>      return float16_is_any_nan(a_);
>  #else
>      uint16_t a = float16_val(a_);
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return (((a >> 9) & 0x3F) == 0x3E) && (a & 0x1FF);
>      } else {
>          return ((a & ~0x8000) >= 0x7C80);
> @@ -338,7 +356,7 @@ int float16_is_signaling_nan(float16 a_, float_status *status)
>      return 0;
>  #else
>      uint16_t a = float16_val(a_);
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return ((a & ~0x8000) >= 0x7C80);
>      } else {
>          return (((a >> 9) & 0x3F) == 0x3E) && (a & 0x1FF);
> @@ -356,7 +374,7 @@ float16 float16_silence_nan(float16 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      g_assert_not_reached();
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return float16_default_nan(status);
>      } else {
>          return a | (1 << 9);
> @@ -375,7 +393,7 @@ int float32_is_quiet_nan(float32 a_, float_status *status)
>      return float32_is_any_nan(a_);
>  #else
>      uint32_t a = float32_val(a_);
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return (((a >> 22) & 0x1FF) == 0x1FE) && (a & 0x003FFFFF);
>      } else {
>          return ((uint32_t)(a << 1) >= 0xFF800000);
> @@ -394,7 +412,7 @@ int float32_is_signaling_nan(float32 a_, float_status *status)
>      return 0;
>  #else
>      uint32_t a = float32_val(a_);
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return ((uint32_t)(a << 1) >= 0xFF800000);
>      } else {
>          return (((a >> 22) & 0x1FF) == 0x1FE) && (a & 0x003FFFFF);
> @@ -412,7 +430,7 @@ float32 float32_silence_nan(float32 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      g_assert_not_reached();
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>  # ifdef TARGET_HPPA
>          a &= ~0x00400000;
>          a |=  0x00200000;
> @@ -651,7 +669,7 @@ static int pickNaNMulAdd(flag aIsQNaN, flag aIsSNaN, flag bIsQNaN, flag bIsSNaN,
>          return 3;
>      }
>
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          /* Prefer sNaN over qNaN, in the a, b, c order. */
>          if (aIsSNaN) {
>              return 0;
> @@ -786,7 +804,7 @@ int float64_is_quiet_nan(float64 a_, float_status *status)
>      return float64_is_any_nan(a_);
>  #else
>      uint64_t a = float64_val(a_);
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return (((a >> 51) & 0xFFF) == 0xFFE)
>              && (a & 0x0007FFFFFFFFFFFFULL);
>      } else {
> @@ -806,7 +824,7 @@ int float64_is_signaling_nan(float64 a_, float_status *status)
>      return 0;
>  #else
>      uint64_t a = float64_val(a_);
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return ((a << 1) >= 0xFFF0000000000000ULL);
>      } else {
>          return (((a >> 51) & 0xFFF) == 0xFFE)
> @@ -825,7 +843,7 @@ float64 float64_silence_nan(float64 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      g_assert_not_reached();
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>  # ifdef TARGET_HPPA
>          a &= ~0x0008000000000000ULL;
>          a |=  0x0004000000000000ULL;
> @@ -942,7 +960,7 @@ int floatx80_is_quiet_nan(floatx80 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      return floatx80_is_any_nan(a);
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          uint64_t aLow;
>
>          aLow = a.low & ~0x4000000000000000ULL;
> @@ -967,7 +985,7 @@ int floatx80_is_signaling_nan(floatx80 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      return 0;
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return ((a.high & 0x7FFF) == 0x7FFF)
>              && ((a.low << 1) >= 0x8000000000000000ULL);
>      } else {
> @@ -991,7 +1009,7 @@ floatx80 floatx80_silence_nan(floatx80 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      g_assert_not_reached();
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return floatx80_default_nan(status);
>      } else {
>          a.low |= LIT64(0xC000000000000000);
> @@ -1105,7 +1123,7 @@ int float128_is_quiet_nan(float128 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      return float128_is_any_nan(a);
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return (((a.high >> 47) & 0xFFFF) == 0xFFFE)
>              && (a.low || (a.high & 0x00007FFFFFFFFFFFULL));
>      } else {
> @@ -1125,7 +1143,7 @@ int float128_is_signaling_nan(float128 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      return 0;
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return ((a.high << 1) >= 0xFFFF000000000000ULL)
>              && (a.low || (a.high & 0x0000FFFFFFFFFFFFULL));
>      } else {
> @@ -1145,7 +1163,7 @@ float128 float128_silence_nan(float128 a, float_status *status)
>  #ifdef NO_SIGNALING_NANS
>      g_assert_not_reached();
>  #else
> -    if (status->snan_bit_is_one) {
> +    if (snan_bit_is_one(status)) {
>          return float128_default_nan(status);
>      } else {
>          a.high |= LIT64(0x0000800000000000);
> diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
> index 4e378cb612..2aae6a89b1 100644
> --- a/include/fpu/softfloat-types.h
> +++ b/include/fpu/softfloat-types.h
> @@ -173,6 +173,7 @@ typedef struct float_status {
>      /* should denormalised inputs go to zero and set the input_denormal flag? */
>      flag flush_inputs_to_zero;
>      flag default_nan_mode;
> +    /* not always used -- see snan_bit_is_one() in softfloat-specialize.h */
>      flag snan_bit_is_one;
>  } float_status;
>
> diff --git a/target/hppa/cpu.c b/target/hppa/cpu.c
> index c261b6b090..00bf444620 100644
> --- a/target/hppa/cpu.c
> +++ b/target/hppa/cpu.c
> @@ -141,7 +141,6 @@ static void hppa_cpu_initfn(Object *obj)
>      cs->env_ptr = env;
>      cs->exception_index = -1;
>      cpu_hppa_loaded_fr0(env);
> -    set_snan_bit_is_one(true, &env->fp_status);
>      cpu_hppa_put_psw(env, PSW_W);
>  }
>
> diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c
> index 9ae418a577..d31a933cbb 100644
> --- a/target/ppc/fpu_helper.c
> +++ b/target/ppc/fpu_helper.c
> @@ -3382,7 +3382,6 @@ void helper_xssqrtqp(CPUPPCState *env, uint32_t opcode)
>              xt.f128 = xb.f128;
>          } else if (float128_is_neg(xb.f128) && !float128_is_zero(xb.f128)) {
>              float_invalid_op_excp(env, POWERPC_EXCP_FP_VXSQRT, 1);
> -            set_snan_bit_is_one(0, &env->fp_status);
>              xt.f128 = float128_default_nan(&env->fp_status);
>          }
>      }
> diff --git a/target/sh4/cpu.c b/target/sh4/cpu.c
> index 541ffc2d97..b9f393b7c7 100644
> --- a/target/sh4/cpu.c
> +++ b/target/sh4/cpu.c
> @@ -71,7 +71,6 @@ static void superh_cpu_reset(CPUState *s)
>      set_flush_to_zero(1, &env->fp_status);
>  #endif
>      set_default_nan_mode(1, &env->fp_status);
> -    set_snan_bit_is_one(1, &env->fp_status);
>  }
>
>  static void superh_cpu_disas_set_info(CPUState *cpu, disassemble_info *info)
> diff --git a/target/unicore32/cpu.c b/target/unicore32/cpu.c
> index 29d160a88d..68f978d80b 100644
> --- a/target/unicore32/cpu.c
> +++ b/target/unicore32/cpu.c
> @@ -70,7 +70,6 @@ static void unicore_ii_cpu_initfn(Object *obj)
>
>      set_feature(env, UC32_HWCAP_CMOV);
>      set_feature(env, UC32_HWCAP_UCF64);
> -    set_snan_bit_is_one(1, &env->ucf64.fp_status);
>  }
>
>  static void uc32_any_cpu_initfn(Object *obj)
> @@ -83,7 +82,6 @@ static void uc32_any_cpu_initfn(Object *obj)
>
>      set_feature(env, UC32_HWCAP_CMOV);
>      set_feature(env, UC32_HWCAP_UCF64);
> -    set_snan_bit_is_one(1, &env->ucf64.fp_status);
>  }
>
>  static void uc32_cpu_realizefn(DeviceState *dev, Error **errp)


--
Alex Bennée