[llvm-commits] [llvm] r133022 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp test/CodeGen/X86/shl_undef.ll

[Duncan Sands]

Hi Chad,

> When pattern matching during instruction selection make sure shl x,1 is not
> converted to add x,x if x is a undef.  add undef, undef does not guarantee
> that the resulting low order bit is zero.
> Fixes<rdar://problem/9453156>  and<rdar://problem/9487392>.

this does not help in the case that x does not appear to be equal to "undef",
so you do this transform, but later optimizers expose that it is in fact
equal to undef.  I don't have a good solution to this.

Ciao, Duncan.