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Re: [Qemu-riscv] [Qemu-devel] [RFC v1 15/23] riscv: tcg-target: Add bran

From: Richard Henderson
Subject: Re: [Qemu-riscv] [Qemu-devel] [RFC v1 15/23] riscv: tcg-target: Add branch and jump instructions
Date: Wed, 21 Nov 2018 08:40:11 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0

On 11/21/18 12:49 AM, Alistair Francis wrote:
> On Fri, Nov 16, 2018 at 1:14 AM Richard Henderson
> <address@hidden> wrote:
>> On 11/15/18 11:36 PM, Alistair Francis wrote:
>>> +static void tcg_out_brcond(TCGContext *s, TCGCond cond, TCGReg arg1,
>>> +                           TCGReg arg2, TCGLabel *l)
>>> +{
>>> +    RISCVInsn op = tcg_brcond_to_riscv[cond].op;
>>> +    bool swap = tcg_brcond_to_riscv[cond].swap;
>>> +
>>> +    tcg_out_opc_branch(s, op, swap ? arg2 : arg1, swap ? arg1 : arg2, 0);
>> You might want to tcg_debug_assert(op != 0) here.
>>> +    if (l->has_value) {
>>> +        reloc_sbimm12(s->code_ptr - 1, l->u.value_ptr);
>> I'm concerned about the conditional branch range.  +-4K isn't much to work
>> with.  The minimum we have for other hosts is +-32K.
>> We have two options: (1) greatly reduce the max size of the TB for this host;
>> (2) be prepared to emit a 2 insn sequence: conditional branch across
>> unconditional branch, with forward branches that turn out to be small patched
>> with a nop.
>> FWIW, the first case would be done via modification of tcg_op_buf_full.  You
>> might have to go as low as 500 opcodes, I'm not sure.
> How do we do option 2?

If l->has_value, just check the actual range.  But of course backward branching
isn't that common in tcg generated code.  Most branches within the TB are short
forward branches, but we also don't know how short is short.

But every TB begins with a test of env->exit_code and a conditional branch to
the end of the block, where we place some code to return to the main loop and
return the pointer to the TB at which we exited.  Thus every TB has a branch
that spans the size of the entire TB.

So, invent (or repurpose) an R_RISCV_FOO value.  It doesn't matter which
because it's private within tcg/riscv/.  Just add some commentary.  (See e.g.
tcg/sparc/ and its use of R_SPARC_13.)

While generating code, emit the conditional branch as normal; leave the unknown
destination 0 for now.  Emit a nop as the second insn.

When resolving R_RISCV_FOO, if the conditional branch is in range, great!  Just
patch it.  If it is out of range, then you need to edit the conditional branch
to reverse the condition (insn ^ (1 << 12)) and branch to pc+8, i.e. over the
next instruction.  Which was a nop during generation, but you will now install
jal r0,dest going to the real destination.


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