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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Hello,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>About a month ago, I submitted a set of patches for review on llvm-commit. The most controversial of the patches, <a href="http://lists.cs.uiuc.edu/pipermail/llvm-commits/attachments/20150622/d104ea71/attachment-0009.obj">http://lists.cs.uiuc.edu/pipermail/llvm-commits/attachments/20150622/d104ea71/attachment-0009.obj</a> deals with the fact that before ARMv8, the rGPR RegClass in Thumb encodings didn’t include SP; but from ARMv8 onwards, it does include it.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>RegClass membership is currently implemented as entirely static, driven by read-only TableGen’d tables and switch blocks encoding the various classes, subclasses, and their relationships. The approach I had taken in my patch was to include ARMGenInstrInfo.inc for a second time, after re-defining rGPRRegClassID to use a different RegClass in the same instructions. Then, Thumb2InstrInfo constructor overrides the MCInstrInfo that it inherited from ARMBaseInstrInfo with the “patched” one. (This “patching”, still, happens at compile-time.)<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>As John rightly noted, this approach looks rather hacky and confusing:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoPlainText style='margin-left:36.0pt'>This isn't "rGPR includes SP" it's "rGPR is #defined to GPRnopc", which makes things really confusing if you're just looking at the .td files to figure out what's what you see instructions that use rGPR, and you see a definition of the rGPR register class, but those instructions may<o:p></o:p></p><p class=MsoPlainText style='margin-left:36.0pt'>actually be redefined to use GPRnopc due to something happening elsewhere that you haven't noticed.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>James added, “Given these instructions are no longer unpredictable the rest of the compiler should know about them too, not just the MC layer, so this is even more of a hack,” – and he suggested instead converting every instruction that takes an rGPR operand into a multiclass of two instructions, one predicated on HasV8Ops, and the other on its inverse. This isn’t practical, however: ARMInstrThumb2.td specifies several hundred individual encodings, InstAliases and match patterns that use rGPR, and all these would need to be manually duplicated.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Right now, we’re trying to come up with a more sensible approach, and perhaps one that could be useful for other targets as well.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Any suggestions? And are there any other cases where the definition of a register class depends on target attributes?<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p></div></body></html>