Pierrick Bouvier <[email protected]> writes:
> target_arch() function will reparse target_name() every time if it was
> not set to a proper SYS_EMU_TARGET_* value (when using
> target-info-stub.c), which is not efficient.
>
> Since we want to preserve the constness of TargetInfo but C doesn't give
> us flexible compile time expressions, we simply set target_arch using a
> static constructor once instead.
>
> This was found when doing changes to virtio_access_is_big_endian()
> function, having an overhead of 50% after switching to runtime checks.
> With this, overhead left is around 3%, due to indirect function
> calls.
>
> Signed-off-by: Pierrick Bouvier <[email protected]>
> ---
> target-info-stub.c | 11 ++++++++++-
> target-info.c | 9 +--------
> 2 files changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/target-info-stub.c b/target-info-stub.c
> index 8392d81e8f8..ff86a02247a 100644
> --- a/target-info-stub.c
> +++ b/target-info-stub.c
> @@ -10,13 +10,14 @@
> #include "qemu/target-info.h"
> #include "qemu/target-info-impl.h"
> #include "hw/core/boards.h"
> +#include "qapi/error.h"
> #include "cpu.h"
>
> /* Validate correct placement of CPUArchState. */
> QEMU_BUILD_BUG_ON(offsetof(ArchCPU, parent_obj) != 0);
> QEMU_BUILD_BUG_ON(offsetof(ArchCPU, env) != sizeof(CPUState));
>
> -static const TargetInfo target_info_stub = {
> +static TargetInfo target_info_stub = {
> .target_name = TARGET_NAME,
> .target_arch = SYS_EMU_TARGET__MAX,
> .long_bits = TARGET_LONG_BITS,
> @@ -29,3 +30,11 @@ const TargetInfo *target_info(void)
> {
> return &target_info_stub;
> }
> +
> +__attribute__((constructor))
> +static void init_target_arch(void)
> +{
> + target_info_stub.target_arch = qapi_enum_parse(&SysEmuTarget_lookup,
> + target_name(), -1,
> + &error_abort);
> +}
This is slightly unclean. Constructors run before main(). If
qapi_enum_parse(..., &error_abort) fails here, error_handle() will
report the unexpected error with error_report() before main() calls
error_init().
See also
From: Markus Armbruster <[email protected]>
Subject: Re: [PATCH v3 02/20] monitor: initialize global data from a
constructor
Date: Thu, 18 Sep 2025 08:30:23 +0200
Message-ID: <[email protected]>
Observation, not objection.
> diff --git a/target-info.c b/target-info.c
> index 24696ff4111..c3c0856d01a 100644
> --- a/target-info.c
> +++ b/target-info.c
> @@ -10,7 +10,6 @@
> #include "qemu/target-info.h"
> #include "qemu/target-info-qapi.h"
> #include "qemu/target-info-impl.h"
> -#include "qapi/error.h"
>
> const char *target_name(void)
> {
> @@ -24,13 +23,7 @@ unsigned target_long_bits(void)
>
> SysEmuTarget target_arch(void)
> {
> - SysEmuTarget arch = target_info()->target_arch;
> -
> - if (arch == SYS_EMU_TARGET__MAX) {
> - arch = qapi_enum_parse(&SysEmuTarget_lookup, target_name(), -1,
> - &error_abort);
> - }
> - return arch;
> + return target_info()->target_arch;
> }
>
> const char *target_cpu_type(void)
