Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only once

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eric Blake (eblake@redhat.com) wrote:
> Add the macro QEMU_TYPEOF() to access __auto_type in new enough
> compilers, while falling back to typeof on older compilers (the
> fallback doesn't handle variable length arrays, but we don't use
> those; it also expands to more text).
> 
> Then use that macro to make MIN/MAX only evaluate their argument
> once; this uses type promotion (by adding to 0) to work around
> the fact that typeof(bitfield) won't compile.  However, we are
> unable to work around gcc refusing to compile ({}) in a constant
> context, even when only used in the dead branch of a
> __builtin_choose_expr(), so we have to provide a second macro
> pair MIN_CONST and MAX_CONST for use when both arguments are
> known to be compile-time constants and where the result must
> also be usable in constant contexts.
> 
> Fix the resulting callsites that compute a compile-time constant
> min or max to use the new macros.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> v2: Force use of our smart macros, and make macro handle bitfields,
> constant expressions, and older compilers [Zoltan]
> ---
>  hw/usb/hcd-xhci.h       |  2 +-
>  include/exec/cpu-defs.h |  2 +-
>  include/qemu/compiler.h | 10 +++++++++
>  include/qemu/osdep.h    | 46 ++++++++++++++++++++++++++++++++++-------
>  migration/qemu-file.c   |  2 +-
>  5 files changed, 51 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/usb/hcd-xhci.h b/hw/usb/hcd-xhci.h
> index fc36a4c787c..10beee9a7b1 100644
> --- a/hw/usb/hcd-xhci.h
> +++ b/hw/usb/hcd-xhci.h
> @@ -210,7 +210,7 @@ struct XHCIState {
>      uint32_t dcbaap_high;
>      uint32_t config;
> 
> -    USBPort  uports[MAX(MAXPORTS_2, MAXPORTS_3)];
> +    USBPort  uports[MAX_CONST(MAXPORTS_2, MAXPORTS_3)];
>      XHCIPort ports[MAXPORTS];
>      XHCISlot slots[MAXSLOTS];
>      uint32_t numports;
> diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
> index 6a60f94a41d..5b2fd46f687 100644
> --- a/include/exec/cpu-defs.h
> +++ b/include/exec/cpu-defs.h
> @@ -90,7 +90,7 @@ typedef uint64_t target_ulong;
>   * of tlb_table inside env (which is non-trivial but not huge).
>   */
>  #define CPU_TLB_BITS                                             \
> -    MIN(8,                                                       \
> +    MIN_CONST(8,                                                 \
>          TCG_TARGET_TLB_DISPLACEMENT_BITS - CPU_TLB_ENTRY_BITS -  \
>          (NB_MMU_MODES <= 1 ? 0 :                                 \
>           NB_MMU_MODES <= 2 ? 1 :                                 \
> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> index 261842beae2..3bf6f68a6b0 100644
> --- a/include/qemu/compiler.h
> +++ b/include/qemu/compiler.h
> @@ -191,4 +191,14 @@
>  #define QEMU_GENERIC9(x, a0, ...) QEMU_GENERIC_IF(x, a0, QEMU_GENERIC8(x, __VA_ARGS__))
>  #define QEMU_GENERIC10(x, a0, ...) QEMU_GENERIC_IF(x, a0, QEMU_GENERIC9(x, __VA_ARGS__))
> 
> +/*
> + * Automatic type deduction, to be used as:
> + * QEMU_TYPEOF(expr) name = expr;
> + */
> +#if QEMU_GNUC_PREREQ(4, 9)
> +# define QEMU_TYPEOF(a) __auto_type
> +#else
> +# define QEMU_TYPEOF(a) typeof(a)
> +#endif
> +
>  #endif /* COMPILER_H */
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 3bf48bcdec0..821ce627f73 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -232,18 +232,48 @@ extern int daemon(int, int);
>  #define SIZE_MAX ((size_t)-1)
>  #endif
> 
> -#ifndef MIN
> -#define MIN(a, b) (((a) < (b)) ? (a) : (b))
> -#endif
> -#ifndef MAX
> -#define MAX(a, b) (((a) > (b)) ? (a) : (b))
> -#endif
> +/*
> + * Two variations of MIN/MAX macros. The first is for runtime use, and
> + * evaluates arguments only once (so it is safe even with side
> + * effects), but will not work in constant contexts (such as array
> + * size declarations).  The second is for compile-time use, where
> + * evaluating arguments twice is safe because the result is going to
> + * be constant anyway.
> + */
> +#undef MIN
> +#define MIN(a, b)                            \
> +    ({                                       \
> +        QEMU_TYPEOF((a) + 0) _a = (a) + 0;   \
> +        QEMU_TYPEOF((b) + 0) _b = (b) + 0;   \
> +        _a < _b ? _a : _b;                   \
> +    })
> +#define MIN_CONST(a, b)                                         \
> +    __builtin_choose_expr(                                      \
> +        __builtin_constant_p(a) && __builtin_constant_p(b),     \
> +        (a) < (b) ? (a) : (b),                                  \
> +        __builtin_unreachable())

Why do these need to be separate macros? Can't you just put the 
non-constant code in what you have as the 'builtin_unreachable' side of
the choose_expr:

#define DMIN(a,b) __builtin_choose_expr(                  \
    __builtin_constant_p(a) && __builtin_constant_p(b),   \
    (a) < (b) ? (a) : (b),                                \
    ({                                       \
        QEMU_TYPEOF((a) + 0) _a = (a) + 0;   \
        QEMU_TYPEOF((b) + 0) _b = (b) + 0;   \
        _a < _b ? _a : _b;                   \
    }))

Dave

> +#undef MAX
> +#define MAX(a, b)                            \
> +    ({                                       \
> +        QEMU_TYPEOF((a) + 0) _a = (a) + 0;   \
> +        QEMU_TYPEOF((b) + 0) _b = (b) + 0;   \
> +        _a > _b ? _a : _b;                   \
> +    })
> +#define MAX_CONST(a, b)                                         \
> +    __builtin_choose_expr(                                      \
> +        __builtin_constant_p(a) && __builtin_constant_p(b),     \
> +        (a) > (b) ? (a) : (b),                                  \
> +        __builtin_unreachable())
> 
>  /* Minimum function that returns zero only iff both values are zero.
>   * Intended for use with unsigned values only. */
>  #ifndef MIN_NON_ZERO
> -#define MIN_NON_ZERO(a, b) ((a) == 0 ? (b) : \
> -                                ((b) == 0 ? (a) : (MIN(a, b))))
> +#define MIN_NON_ZERO(a, b)                              \
> +    ({                                                  \
> +        QEMU_TYPEOF((a) + 0) _a = (a) + 0;              \
> +        QEMU_TYPEOF((b) + 0) _b = (b) + 0;              \
> +        _a == 0 ? _b : (_b == 0 || _b > _a) ? _a : _b;  \
> +    })
>  #endif
> 
>  /* Round number down to multiple */
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 977b9ae07c1..9746cbec0f3 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -31,7 +31,7 @@
>  #include "trace.h"
> 
>  #define IO_BUF_SIZE 32768
> -#define MAX_IOV_SIZE MIN(IOV_MAX, 64)
> +#define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64)
> 
>  struct QEMUFile {
>      const QEMUFileOps *ops;
> -- 
> 2.20.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK