llvm dev - Feb 2007 - [LLVMdev] Misc optimization issue

This is from 'early JIT tests' thread on comp.lang.misc.

Given :-

int ltst(int x)
{
int i, j;

j=0;
for(i=0; i<x; i++)j++;

return(j);
}

This is from the online LLVM compiler. AND It does not seem to optimize it down
!

; ModuleID = '/tmp/webcompile/_24843_0.bc'
target datalayout = "e-p:32:32"
target endian = little
target pointersize = 32
target triple = "i686-pc-linux-gnu"

implementation   ; Functions:

int %ltst(int %x) {
entry:
%x = cast int %x to uint ; <uint> [#uses=1]
%tmp13 = setgt int %x, 0 ; <bool> [#uses=1]
br bool %tmp13, label %bb, label %bb7

bb:                                                 ; preds = %bb, %entry
%indvar = phi uint [ 0, %entry ], [ %indvar.next, %bb ] ; <uint> [#uses=2]
%i.0.0 = cast uint %indvar to int                 ; <int> [#uses=1]
%tmp1 = add int %i.0.0, 1                 ; <int> [#uses=1]
%indvar.next = add uint %indvar, 1            ; <uint> [#uses=2]
%exitcond = seteq uint %indvar.next, %x ; <bool> [#uses=1]
br bool %exitcond, label %bb7, label %bb

bb7:                                                 ; preds = %bb, %entry
%j.0.1 = phi int [ 0, %entry ], [ %tmp1, %bb ] ; <int> [#uses=1]
ret int %j.0.1
}

Aaron
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On Mon, 5 Feb 2007, Aaron Gray wrote:
> This is from 'early JIT tests' thread on comp.lang.misc.
Interesting, that's a great catch.  Because of the particular 
configuration of phi nodes in this case, we don't catch it.  I filed
http://llvm.org/PR1179 to track this.  Note that this is due to the LLVM 
loop optimizer, it has nothing to do with the JIT. :)

Thanks,

-Chris
> Given :-
>
> int ltst(int x)
> {
> int i, j;
>
> j=0;
> for(i=0; i<x; i++)j++;
>
> return(j);
> }
>
> This is from the online LLVM compiler. AND It does not seem to optimize it
down !
>
> ; ModuleID = '/tmp/webcompile/_24843_0.bc'
> target datalayout = "e-p:32:32"
> target endian = little
> target pointersize = 32
> target triple = "i686-pc-linux-gnu"
>
> implementation   ; Functions:
>
> int %ltst(int %x) {
> entry:
> %x = cast int %x to uint ; <uint> [#uses=1]
> %tmp13 = setgt int %x, 0 ; <bool> [#uses=1]
> br bool %tmp13, label %bb, label %bb7
>
> bb:                                                 ; preds = %bb, %entry
> %indvar = phi uint [ 0, %entry ], [ %indvar.next, %bb ] ; <uint>
[#uses=2]
> %i.0.0 = cast uint %indvar to int                 ; <int> [#uses=1]
> %tmp1 = add int %i.0.0, 1                 ; <int> [#uses=1]
> %indvar.next = add uint %indvar, 1            ; <uint> [#uses=2]
> %exitcond = seteq uint %indvar.next, %x ; <bool> [#uses=1]
> br bool %exitcond, label %bb7, label %bb
>
> bb7:                                                 ; preds = %bb, %entry
> %j.0.1 = phi int [ 0, %entry ], [ %tmp1, %bb ] ; <int> [#uses=1]
> ret int %j.0.1
> }
>
> Aaron
>
-Chris

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