[LLVMdev] DeadStoreElimination: do better without TargetData

[Chris Lattner]

On Nov 4, 2009, at 10:13 AM, Hans Wennborg wrote:

> Sorry, I admit my e-mail was a bit unclear.
> Here is an example:
>
> declare void @foo()
>
> define void @f(i32* noalias %p) {
>        store i32 1, i32* %p; <-- This is dead
>        call void @foo()
>        store i32 2, i32 *%p
>        ret void
> }
>
> When run through ./llvm-as test.ll -o - | ./opt -O3 -S
> the dead store is not removed by the DSE pass.
> (The call to @foo is there to prevent InstCombine from folding the  
> store instructions into one.)
>
> This is because DSE relies on TargetData to find out about pointer  
> sizes. Apparently, there was some default value for this before, but  
> when running with top of the tree today, it turned out that TD=NULL  
> in the DSE code.
>
> We discussed this in the IRC channel, and someone pointed out that  
> the more the pass can do without having info about the target, the  
> better --  hence the patch.
>
> With the patch, DSE will at least see if the data types are equal,  
> even though it doesn't have information about their sizes. This is  
> enough for handling the example above, and probably many others as  
> well.

Thanks, applied here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20091102/090396.html

I'd appreciate it if you could resend your other patches in patch  
format like this, attached to the email with the testcase using  
filecheck.  This commit should be a good example.  Thanks for helping  
improve this area!

-Chris