[llvm-commits] [llvm] r114599 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp test/CodeGen/X86/bit-test-shift.ll
clattner at apple.com
Thu Sep 23 02:12:33 CDT 2010
On Sep 22, 2010, at 11:35 PM, Nick Lewycky wrote:
> 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.
>> can't you do this at the IR level? At the IR level, you can change
>> %s = select i1 %cond, i32 %a, i32 0
>> %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.
Yes, I agree. The canonical form at the IR level should be a select.
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