Need a machine code LICM pass

[llvmbot]

Bugzilla Link	893
Resolution	FIXED
Resolved on	Apr 22, 2009 04:28
Version	1.0
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@isanbard

Extended Description

On some architectures (and PIC modes) using the address of a global variable is
not cheap (sometimes involving a load). Thus the global should be hoisted out
of loops. This is probably true of large constants that the target will wind up
spilling to the constant pool also.
Thus LICM (and probably instcombine and such) should be aware of this (informed
by target data I'm sure).

consider:
%GV = internal global int 0

...
cond_true:
%b = call foo(int* %GV,...)
br bool %b, %cond_true, %cond_false

here the passing of GV causes an extra load each iteration of the loop. This is
bad.

[llvmbot]

I assumed in that example that foo was external, thus ipsccp would not help.

[lattner]

Totally agreed.  In fact, it's more general than that, there are lots of things that lower to multiple ops at
the machine code level.  We just want a very simple LICM pass that works on machine instrs without side-
effects.

-Chris

[lattner]

*** Bug llvm/llvm-bugzilla-archive#1298 has been marked as a duplicate of this bug. ***

[lattner]

When Owen's dominator work lands, this should be possible.

[llvmbot]

This has been done for a while.