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Re: [Qemu-devel] Re: Compile files only once: some planning
From: |
Richard Henderson |
Subject: |
Re: [Qemu-devel] Re: Compile files only once: some planning |
Date: |
Wed, 24 Mar 2010 10:07:36 -0700 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.8) Gecko/20100301 Fedora/3.0.3-1.fc12 Thunderbird/3.0.3 |
On 03/24/2010 07:45 AM, Paolo Bonzini wrote:
> On 03/24/2010 12:19 PM, Richard Henderson wrote:
>> On 03/24/2010 02:47 AM, Paolo Bonzini wrote:
>>> 1) make CPUState define only common fields. Include CPUState at the
>>> beginning of each per-target CPUXYZState.
>>
>> Irritatingly, the common fields contain quite big TLBs. And the
>> offsets from the start of env affect the compactness of the code
>> generated from TCG. We really really want the general registers
>> to come first to make sure that those offsets fit the host's
>> reg+offset addressing mode.
>
> What about adding a 512-bytes (or more) block or something like that at
> the beginning of CPUState with a union, so you can put the per-target
> stuff there?
I think that would be confusing.
What might be just as good (although possibly just as confusing)
is to move the big members into a different structure. E.g.
struct CPUSmallCommonState
{
// most of the stuff from CPU_COMMON.
// sorted for some thought of padding elimination. ;-)
};
struct CPULargeCommonState
{
CPUTLBEntry tlb_table[NB_MMU_MODES][CPU_TLB_SIZE];
target_phys_addr_t iotlb[NB_MMU_MODES][CPU_TLB_SIZE];
struct TranslationBlock *tb_jmp_cache[TB_JMP_CACHE_SIZE];
jmp_buf jmp_env;
};
struct CPUXYZSmallState
{
CPUSmallCommonState common_s;
// the rest of the cpu-specific stuff.
};
struct CPUXYZLargeState
{
CPUXYZSmallState s;
CPUBigCommonState common_l;
};
extern int cpu_large_state_offset = offsetof(CPUXYZLargeState, common_l);
Now. If you're compiling a file for which cpu-specific code is ok:
register CPUXYZLargeState *env __asm__(AREG0);
#define ENV_SMALL_COMMON_STATE (&env->s.common_s)
#define ENV_LARGE_COMMON_STATE (&env->common_l)
If you're compiling a file which is supposed to be independant of cpu:
register CPUSmallCommonState *env __asm__(AREG0);
#define ENV_SMALL_COMMON_STATE (env)
#define ENV_LARGE_COMMON_STATE ((CPULargeCommonState *)((char *)env +
cpu_large_state_offset))
For the gcc-compiled code, the addition of the cpu_large_state_offset
is probably more or less on par in efficiency with indirection. But
for TCG generated code, the variable read happens at code generation
time, which means we *still* have a constant in the generated code.
r~
- Re: [Qemu-devel] Re: Compile files only once: some planning, (continued)
Re: [Qemu-devel] Re: Compile files only once: some planning, Jamie Lokier, 2010/03/24
[Qemu-devel] Re: Compile files only once: some planning, Paolo Bonzini, 2010/03/24
Re: [Qemu-devel] Re: Compile files only once: some planning,
Richard Henderson <=
Re: [Qemu-devel] Re: Compile files only once: some planning, Richard Henderson, 2010/03/24
Re: [Qemu-devel] Re: Compile files only once: some planning, Blue Swirl, 2010/03/24
[Qemu-devel] Re: Compile files only once: some planning, Michael S. Tsirkin, 2010/03/24