[llvm-dev] Instruction selection for 'load' based on static vs. dynamic data

[Dr. ERDI Gergo via llvm-dev]

Hi,

I've been looking at the new AVR backend lately, and one issue I've
found is that all 'load' IR instructions are matched using the 'ld'
AVR instruction, including 'load's for lookup tables generated from
switches.

On the AVR architecture, RAM and the program image are in completely
separated namespaces. There's a distinct 'lpm' (Load from Program
Memory) instruction that can load data from .text. So the nice way of
getting this to work is to select 'lpm' for those switch lookup table
'load's, thereby avoiding the memory footprint and the runtime cost of
copying the tables to RAM at startup. However, we still need to select
'ld' for other 'load's.

To give a concrete example, here's an LLVM IR snippet that should be
lowered to 'lpm':

     @switch.table = private unnamed_addr constant [15 x i8] 
c"\01\01\01\01\01\00\00\00\00\00\00\00\00\00\01"

     switch.lookup:                                    ; preds = %start
       %2 = sext i8 %0 to i16
       %switch.gep = getelementptr inbounds [15 x i8], [15 x i8]* 
@switch.table, i16 0, i16 %2
       %switch.load = load i8, i8* %switch.gep, align 1
       br label %bb8

and here's one that should be lowered to 'ld':

     %S = type { i8 }

     start:
       %1 = getelementptr inbounds %S, %S* %0, i16 0, i32 0
       %2 = load i8, i8* %1, align 1
       ret i8 %2
     }

So my question is, what kind of pattern matching can I do in my 'lpm'
and 'ld' instructions that it would be able to make a distinction
about these two usecases?


Thanks,
         Gergo

-- 

   .--= ULLA! =-----------------.
    \     http://gergo.erdi.hu   \
     `---= gergo at erdi.hu =-------'
Have you heard about the new OO COBOL language? "ADD 1 TO COBOL GIVING COBOL"