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r199628
2014 Sep 26
2
[LLVMdev] Use of custom operations after DAG legalization
...ible, my custom-lowering returns SDValue() and so falls back onto the default expansion, which is to generate i16 SMUL_LOHI and use only the low result. This worked fine when I was using 3.4 but in 3.5.0, the DAG Combiner replaces a SMUL_LOHI whose high result is unused with a MUL. I traced this to r199678 in which the DAG Combiner was changed specifically to allow this replacement when MUL is either Legal or Custom. In 3.4, it was only permitted if MUL was Legal. For my target, the 3.5.0 behaviour isn't acceptable because the subsequent instruction selection will fail as i16 MUL isn't legal....