[llvm-commits] [llvm] r42075 - in /llvm/trunk: lib/Transforms/Scalar/LICM.cpp test/Transforms/LICM/2007-09-17-PrompteValue.ll

[Dan Gohman]

> It is not safe, because cond_true.i is a dead basic-block. The  
> question is -- Is it LICM's responsibility to evaluate conditions and  
> identify dead code or let other passes handle this earlier?. For  
> example here, instruction combiner will fold comparison and cfg  
> simplifier will take care of cond_true.i before LICM is executed in  
> standard pass sequence.

Ok, so let's tweak the testcase again:

define i32 @foo(%struct.decision* %p, %struct.decision* %q) {
entry:
       br label %blah.i

blah.i:                ; preds = %cond_true.i, %entry
       %tmp3.i = icmp eq %struct.decision* null, %q          ; <i1> [#uses=1]
       br i1 %tmp3.i, label %clear_modes.exit, label %cond_true.i

cond_true.i:           ; preds = %blah.i
       %tmp1.i = getelementptr %struct.decision* %p, i32 0, i32 0            ; <i8*> [#uses=1]
       store i8 0, i8* %tmp1.i
       br label %blah.i

clear_modes.exit:              ; preds = %blah.i
       call void @exit( i32 0 )
       unreachable
}

declare void @exit(i32)


It's no longer possible for any part of the optimizer to prove the loop
is dead. LICM is still putting a load of %tmp1.i in the entry block for
this test, and that's unsafe.

Dan

-- 
Dan Gohman, Cray Inc.