[LLVMdev] LSR pass

[Jonas Paulsson]

Hi,

The target supports indexing by register or immediate. Multiplications are not supported by any load / store instructions. Would it be possible to make LSR aware of this?

Thanks,

Jonas Paulsson


-----Original Message-----
From: Hal Finkel [mailto:hfinkel at anl.gov] 
Sent: Saturday, December 01, 2012 5:59 AM
To: Jonas Paulsson
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] LSR pass

----- Original Message -----
> From: "Jonas Paulsson" <jonas.paulsson at ericsson.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Monday, November 26, 2012 1:40:24 PM
> Subject: [LLVMdev] LSR pass
> 
> 
> 
> 
> 
> Hi,
> 
> 
> 
> I would like some help regarding the LSR pass. It seems that it likes
> to duplicate address calculations as in the case above, which is
> highly undesirable on my target.
> 
> 
> 
> I wonder if there is any way to tell LSR to not duplicate the code in
> cases like this? Or could I perhaps run CSE after LSR again?
> 
> What is the logic behind this transformation? It seems that a LSR
> pass should not insert a multiplication, generally..?

I believe that the general logic behind this is that on many targets these extra instructions are absorbed into the 'addressing mode' of the user instructions. Does your target support any non-trivial addressing modes?

 -Hal

> 
> 
> 
> Thanks,
> 
> Jonas
> 
> 
> 
> 
> 
> 
> 
> %_tmp44 = ptrtoint i16* par1 to i16
> 
> %_tmp51 = ptrtoint i16* par2 to i16
> 
> ...
> 
> inside loop:
> 
> *** IR Dump After Canonicalize natural loops ***
> 
> bb7: (header) ; preds = %bb7.lr.ph, %bb11
> 
> 
> 
> %_tmp39 = sub i16 %_tmp35, %_tmp38
> 
> %2 = mul i16 %_tmp39, -10
> 
> %_tmp41 = add i16 %2, %subframeCount.12.014
> 
> 
> 
> %_tmp45 = add i16 %_tmp41, %_tmp44
> 
> %_tmp46 = inttoptr i16 %_tmp45 to i16*
> 
> %_tmp47 = load i16* %_tmp46, align 1
> 
> 
> 
> 
> 
> bb8: ; preds = %bb7
> 
> %_tmp52 = add i16 %_tmp41, %_tmp51
> 
> %_tmp53 = inttoptr i16 %_tmp52 to i16*
> 
> %_tmp54 = load i16* %_tmp53, align 1
> 
> 
> 
> ...
> 
> br i1 %_tmp64, label %bb7, label %bb13.loopexit
> 
> (latch)
> 
> 
> 
> *** IR Dump After Loop Strength Reduction ***
> 
> bb7: ; preds = %bb7.lr.ph, %bb11
> 
> 
> 
> %_tmp39 = sub i16 %_tmp35, %_tmp38
> 
> %2 = mul i16 %_tmp39, -10
> 
> %3 = add i16 %_tmp44, %subframeCount.12.014
> 
> %4 = add i16 %3, %2
> 
> %_tmp46 = inttoptr i16 %4 to i16*
> 
> %_tmp47 = load i16* %_tmp46, align 1
> 
> 
> 
> bb8: ; preds = %bb7
> 
> %5 = sub i16 %_tmp35, %_tmp38
> 
> %6 = mul i16 %5, -10
> 
> %7 = add i16 %_tmp51, %subframeCount.12.014
> 
> %8 = add i16 %7, %6
> 
> %_tmp53 = inttoptr i16 %8 to i16*
> 
> %_tmp54 = load i16* %_tmp53, align 1
> 
> 
> 
> ...
> 
> br i1 %_tmp64, label %bb7, label %bb13.loopexit
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> 

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory