[llvm-commits] [llvm] r114599 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp test/CodeGen/X86/bit-test-shift.ll

Nick Lewycky nicholas at mxc.ca
Wed Sep 22 23:35:17 PDT 2010

Duncan Sands wrote:
> Hi Owen,
>> A select between a constant and zero, when fed by a bit test, can be efficiently
>> lowered using a series of shifts.
>> Fixes<rdar://problem/8285015>.
> can't you do this at the IR level?  At the IR level, you can change
>     %s = select i1 %cond, i32 %a, i32 0
> into
>     %cond.1 = sext i1 %cond to i32
>     %s = and i32 %a, %cond.1
> which could be a win depending on where %cond comes from.

I would much rather we turned chains of expressions on i1 into a single 
statement, then taught the backend how to do efficient construction of 
the right values out of the select statement.


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