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Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only
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From: |
Dr. David Alan Gilbert |
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Subject: |
Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only once |
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Date: |
Mon, 7 Jan 2019 09:49:28 +0000 |
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User-agent: |
Mutt/1.10.1 (2018-07-13) |
* Eric Blake (address@hidden) 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 <address@hidden>
>
> ---
> 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 / address@hidden / Manchester, UK