[LLVMdev] "Machine LICM" for Constants?

[Evan Cheng]

Yes machine-licm can and should hoist constant materialization instructions out of the loop. If it's not doing that, it's probably because the target is not modeling the instruction correctly. I would walk through MachineLICM::IsLoopInvariantInst() in the debugger to figure it out. You can also try compiling the same bitcode for a target like ARM or X86 as a comparison.

Evan

On Mar 7, 2012, at 10:38 AM, Matt Johnson wrote:

> Hi All,
>     I work on a backend for a target similar to Mips, where large 
> immediates are loaded into registers with 2 instructions, 1 to load the 
> MSBits and 1 to load the LSBits.  I've noticed a recurring pattern 
> where, despite low register pressure, these constants will be 
> rematerialized in every iteration of a loop, rather than being hoisted.  
> Here's an example using the mips-unknown-unknown target and Clang/LLVM 
> HEAD.  From newlib's implementation of strncat:
> 
> #define DETECTNULL(X) (((X) - 0x01010101) & ~(X) & 0x80808080)
> while (!DETECTNULL (*aligned_s1))
>   aligned_s1++;
> 
> This loop gets lowered under -O3 to:
> 
> $BB0_5:
>   lui $3, 32896
>   lui $7, 65278
>   ori $3, $3, 32896 ###### Materialize 0x80808080
>   lw  $8, 4($2)
>   nop
>   and $9, $8, $3
>   ori $7, $7, 65279 ###### Materialize -(0x01010101)
>   addiu $2, $2, 4
>   xor $3, $9, $3
>   addu  $7, $8, $7
>   and $3, $3, $7
>   beq $3, $zero, $BB0_5
> 
> 
> There are a ton of unused caller-saved registers in this small function, 
> so I expected the constant materialization to be hoisted out of the 
> tight loop.  I'm still learning about the new register allocator and am 
> not immediately able to make sense of its debug output (and the 
> 'problem' may be elsewhere in any case).  I'm happy to post the results 
> of -debug-only regalloc if they're useful.
> 
> Is my desire to hoist the constants out of the loop reasonable?  Is 
> there something I can do (hints or passes in my backend, clang/opt flag, 
> etc.) to make this happen today?  If not, what is the root cause?  Maybe 
> there's no way to hoist things out of a loop once IR is lowered into a 
> SelectionDAG?
> 
> Thanks,
> Matt
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev