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Re: [Qemu-devel] [PATCH] tcg-i386: Improve cmov detection
From: |
Aurelien Jarno |
Subject: |
Re: [Qemu-devel] [PATCH] tcg-i386: Improve cmov detection |
Date: |
Sun, 25 Nov 2012 14:44:18 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Sat, Nov 24, 2012 at 09:39:35AM -0800, Richard Henderson wrote:
> In addition to better compile-time detection, perform runtime detection.
>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
> tcg/i386/tcg-target.c | 34 +++++++++++++++++++++++++++++++++-
> tcg/i386/tcg-target.h | 5 -----
> 2 files changed, 33 insertions(+), 6 deletions(-)
>
> Yall are right that there's no particularly good method with which
> to detect i686 *or later*, and thus cmov support, in gcc. If one
> uses -march=native with any processor made in the last 5 years,
> one will have at least SSE1 support. So we can reasonably use that
> as a clue.
>
> To fill in the holes, we can do the check at runtime. That does
> involve a tiny amount of runtime overhead, testing a global variable.
> I suspect that this is overhead is unmeasurable.
If this overhead is unmesurable, and I think it is something true, I
think it would be better to just always use that on i386 (but not on
x86_64) instead of having a complex compile time detection that could
fail.
Otherwise the patch looks fine.
> diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
> index 6f3ad3c..b333b46 100644
> --- a/tcg/i386/tcg-target.c
> +++ b/tcg/i386/tcg-target.c
> @@ -97,6 +97,20 @@ static const int tcg_target_call_oarg_regs[] = {
> # define TCG_REG_L1 TCG_REG_EDX
> #endif
>
> +/* Attempt to determine at compile-time whether the compiler assumes that
> + cmov is available. We get 64-bit for free. P6 (i686) and later include
> + support for cmov, but there is no one preprocessor define that determines
> + this. Assume that all processors that include sse also support cmov, so
> + that we sorta future-proof this test against new preprocessor defines. */
> +#include <cpuid.h>
> +#if (TCG_TARGET_REG_BITS == 64 \
> + || defined(__i686__) || defined(__pentium4__) \
> + || defined(__athlon__) || defined(__SSE__))
> +# define have_cmov 1
> +#else
> +static bool have_cmov;
> +#endif
> +
> static uint8_t *tb_ret_addr;
>
> static void patch_reloc(uint8_t *code_ptr, int type,
> @@ -943,7 +957,14 @@ static void tcg_out_movcond32(TCGContext *s, TCGCond
> cond, TCGArg dest,
> TCGArg v1)
> {
> tcg_out_cmp(s, c1, c2, const_c2, 0);
> - tcg_out_modrm(s, OPC_CMOVCC | tcg_cond_to_jcc[cond], dest, v1);
> + if (have_cmov) {
> + tcg_out_modrm(s, OPC_CMOVCC | tcg_cond_to_jcc[cond], dest, v1);
> + } else {
> + int over = gen_new_label();
> + tcg_out_jxx(s, tcg_cond_to_jcc[tcg_invert_cond(cond)], over, 1);
> + tcg_out_mov(s, TCG_TYPE_I32, dest, v1);
> + tcg_out_label(s, over, s->code_ptr);
> + }
> }
>
> #if TCG_TARGET_REG_BITS == 64
> @@ -2243,6 +2264,17 @@ static void tcg_target_qemu_prologue(TCGContext *s)
>
> static void tcg_target_init(TCGContext *s)
> {
> + /* If we could not determine cmov availablity at compile time, perform
> + the check at runtime. 99% certainty that we're running on hardware
> + that supports cmov, but we still need to check. In case cmov is not
> + available, we'll use a small forward branch. */
> +#ifndef have_cmov
> + {
> + unsigned a, b, c, d;
> + have_cmov = (__get_cpuid(1, &a, &b, &c, &d) && (d & bit_CMOV));
> + }
> +#endif
> +
> #if !defined(CONFIG_USER_ONLY)
> /* fail safe */
> if ((1 << CPU_TLB_ENTRY_BITS) != sizeof(CPUTLBEntry))
> diff --git a/tcg/i386/tcg-target.h b/tcg/i386/tcg-target.h
> index dbc6756..450078b 100644
> --- a/tcg/i386/tcg-target.h
> +++ b/tcg/i386/tcg-target.h
> @@ -90,12 +90,7 @@ typedef enum {
> #define TCG_TARGET_HAS_nand_i32 0
> #define TCG_TARGET_HAS_nor_i32 0
> #define TCG_TARGET_HAS_deposit_i32 1
> -#if defined(__x86_64__) || defined(__i686__)
> -/* Use cmov only if the compiler is already doing so. */
> #define TCG_TARGET_HAS_movcond_i32 1
> -#else
> -#define TCG_TARGET_HAS_movcond_i32 0
> -#endif
>
> #if TCG_TARGET_REG_BITS == 64
> #define TCG_TARGET_HAS_div2_i64 1
> --
> 1.7.11.7
>
>
--
Aurelien Jarno GPG: 1024D/F1BCDB73
address@hidden http://www.aurel32.net
- Re: [Qemu-devel] [PATCH] qemu-timer: Don't use RDTSC on 386s and 486s, (continued)
- Re: [Qemu-devel] [PATCH] qemu-timer: Don't use RDTSC on 386s and 486s, Paolo Bonzini, 2012/11/23
- Re: [Qemu-devel] [PATCH] qemu-timer: Don't use RDTSC on 386s and 486s, Peter Maydell, 2012/11/23
- Re: [Qemu-devel] [PATCH] qemu-timer: Don't use RDTSC on 386s and 486s, Jamie Lokier, 2012/11/23
- [Qemu-devel] [PATCH] tcg-i386: Improve cmov detection, Richard Henderson, 2012/11/24
- Re: [Qemu-devel] [PATCH] tcg-i386: Improve cmov detection, Peter Maydell, 2012/11/24
- Re: [Qemu-devel] [PATCH] tcg-i386: Improve cmov detection, Richard Henderson, 2012/11/26
- Re: [Qemu-devel] [PATCH] tcg-i386: Improve cmov detection,
Aurelien Jarno <=