[LLVMdev] Not enough optimisations in the SelectionDAG phase?

Matt Johnson
Sun, 29 Apr 2012 19:34:53 -0000

On 04/29/2012 01:19 PM, Evan Cheng wrote:
> On Apr 24, 2012, at 11:48 PM, Fan Dawei wrote:
>
>> For the following code fragment,
>>
>> ; :27 ; preds = %27,
%entry
>>    %28 = load volatile i32* inttoptr (i64 2149581832 to i32*), align 8
>>    %29 = icmp slt i32 %28, 0
>>    br i1 %29, label %27, label %loop.exit
>>
>> loop.exit:                                  ; preds = %27
>>
>> llc will generate following MIPS code,
>>
>> $BB0_1:
>>    lui $3, 32800
>>    ori $3, $3, 1032
>>    lw  $3, 0($3)
>>    bltz  $3, $BB0_1
>>    nop
>> # BB#2:
>>
>>
>> The two operation lui and ori which are used to calculate memory
address actually are loop invariants. They supposed to be moved out of the loop.
I thought it might be a limitation of the MIPS backend.  Then I tried the ARM
backend,
>>
>>   .LBB1_1:
>>    ldr r2, .LCPI1_2
>>    ldr r2, [r2]
>>    cmp r2, #0
>>    blt .LBB1_1
>> @ BB#2:
>>
>> The first ldr instruction is to load the address from constant pool. It
also should be outside the loop.
>>
>> I'm not sure if this is because of the optimisations are not enough
in the common SelectionDAG optimisation phase, or  should this kind of
optimisation be implemented by the SelectionDAG instruction lowering phase for
each target?
I had a mailing list thread on this exact topic last month (see
http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-March/048076.html**).
The underlying cause is that lui and ori are both 'cheap' instructions.
It used to be that cheap instructions would not get hoisted at all
during Machine LICM.
There was a patch a couple weeks back (r154455) that was a bit more
aggressive and will hoist cheap instructions if they don't increase
register pressure, but it doesn't help us in this case because lui/ori
are a pair of dependent ori instructions.  There is a chicken-and-egg
problem where neither can be hoisted without the other, and MachineLICM
is not aggressive enough to recognize chains of dependent,
loop-invariant cheap instructions.
At the time, the advice was to implement a PseudoInstruction for lui+ori
and lower it in a C++ pass, as is done in ARM (see MOVi32imm in
ARMInstrInfo.td and ARMExpandPseudoInsts.cpp).
I did this for my target and it worked fine, so MIPS could do the same.
To me, that solution isn't too satisfying because you have to do this
for every multi-instruction TableGen pattern to get them hoisted out of
loops, but the philosophy seems to be to keep MachineLICM simple.
-Matt

> SelectionDAG doesn't do LICM. Are you running machine-licm pass?
>
> Evan
>
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