[llvm-commits] [llvm] r89356 - /llvm/trunk/lib/CodeGen/RegAllocLinearScan.cpp

[Evan Cheng]

On Nov 19, 2009, at 1:10 PM, David Greene wrote:

> On Thursday 19 November 2009 13:36, Evan Cheng wrote:
>> On Nov 19, 2009, at 7:55 AM, David Greene wrote:
>>> +    // The queue of recently-used registers.
>>> +    SmallVector<unsigned, 3> RecentRegs;
>> 
>> Why the odd sized vector?
> 
> Why not?  We have the queue sized to three in our code.  I can
> change it if you'd like.  It's an arbitrary choice.

It seems strange.

> 
>>> +
>>> +    // Record that we just picked this register.
>>> +    void recordRecentlyUsed(unsigned reg) {
>>> +      assert(reg != 0 && "Recently used register is NOREG!");
>>> +      if (!RecentRegs.empty()) {
>>> +        std::copy(RecentRegs.begin() + 1, RecentRegs.end(),
>>> RecentRegs.begin()); +        RecentRegs.back() = reg;
>> 
>> This seems extremely slow? Why not pop front and push back? Or just use a
>> fixed size array and keep rotating the begin and end pointers?
> 
> pop_front on a vector is no faster than the copy and might actually be slower.
> Perhaps this should be a list<> or deque<>.  What do you think?

deque is slow. Why not just a fixed sized array and rotate the head and tail pointers?

Evan

> 
>                                    -Dave
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