[LLVMdev] introducing sign extending halfword loads into the LLVM IR

[Justin Holewinski]

Instruction selection happens on a different IR: SelectionDAG.  In this IR,
there are sign-extending loads that the IR converter will use, and are used
for example to load 8/16-bit values into 32-bit registers (with sign or
zero extension).  Any optimizations performed during codegen will be in
this representation, or even MachineInstr form, which is post-isel and any
sign-extension information will already be folded into the machine
instruction.

The problem with doing machine-level analysis on LLVM IR is that there is
no guarantee that LLVM IR will be an accurate representation of what the
final code will look like instruction-wise.  LLVM IR can express many
operations that are not legal for a given target and so must be expanded
into more than one operation.  Or the target may support combination
instructions that allow multiple LLVM IR instructions to be folded into one
machine instruction.  All of these are handled during SelectionDAG
generation and instruction selection, and complicate LLVM IR-level analysis
of machine-level characteristics.  For the load example you give, on any
architecture that supports sign-extending loads, the load and sext will be
combined into a single instruction (a "load ... <sext from i16>" in
SelectionDAG).

We could definitely try to capture more of the optimization cases you
mention, but I'm not sure adding a sign-extending load to the IR would buy
us much.  In what cases would a front-end choose to generate a 16-bit load
sign-extended to i32 instead of an i16 load?  This seems like it would only
generate ambiguity.  Generally, we don't extend the core IR if something is
already expressible.  For what it's worth, the max detection could fairly
easily be done in a back-end isel pattern.


On Mon, Jan 21, 2013 at 5:32 AM, Bjorn De Sutter <
bjorn.desutter at elis.ugent.be> wrote:

> Hi all,
>
> when compiling code like
>
> short ptr * = some_address;
> int val;
>
> val = *ptr;
> if (val>2047)
>    val = 2047;
> else if (val<-2048)
>    val = -2048.
> // other things done that require val to be an int ...
>
> The load operation is represented by a load and a sign extension operation
> in the LLVM IR. On most target architectures, there exist signed halfword
> load instructions, so the load and sign extension are effectively
> translated into a single instruction during instruction selection.
> Nonetheless, this sign extension operation in the IR prohibits a lot of
> optimizations:
>
> - it counts as an instruction in heuristics based on instruction counts
> (such as SimplifyCFG); as a result some simplifications are not performed;
> - the sign extension operation gets combined with other operations. In the
> example, the sign extension gets combined with the 32-bit comparison
> implementing the val>-2048, resulting in a 16-bit comparison on the
> non-extended value; the result is a comparison operation on 16-bit
> operands, followed by a select operation on 32-bit operands:
>
>  %0 = load i16* %arrayidx2, align 2, !dbg !502
>   %conv = sext i16 %0 to i32, !dbg !502
>   %cmp16 = icmp sgt i16 %0, 2047, !dbg !510
>   br i1 %cmp16, label %if.end23, label %if.else, !dbg !510
>
> if.else:                                          ; preds = %for.body
>   %cmp19 = icmp slt i16 %0, -2048, !dbg !511
>          <--- 16-bit comparison
>   %.conv = select i1 %cmp19, i32 -2048, i32 %conv, !dbg !511          <---
> 32-bit select
>   br label %if.end23, !dbg !511
>
> if.end23:                                         ; preds = %if.else,
> %for.body
>   %val1.0 = phi i32 [ 2047, %for.body ], [ %.conv, %if.else ]
>   %conv24 = trunc i32 %val1.0 to i16, !dbg !512
>   store i16 %conv24, i16* %arrayidx2, align 2, !dbg !512
>
> The problem with this is that during instruction selection, the pair of
> comparison and select is no longer recognized as max operation because the
> operands of the two operations are not the same.
>
> It seems to me that these and other limitations on the applied
> optimizations could easily be avoided by introducing a sign-extending
> halfword load operation into the IR.
>
> Any ideas on this? And how big of an effort that might be?
>
> Thanks,
>
> Bjorn De Sutter
> Computer Systems Lab
> Ghent University
>
>
>
>
>
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>



-- 

Thanks,

Justin Holewinski
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