[llvm-dev] isSSA computation in MIR parser

Verena Beckham via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 26 04:50:12 PDT 2017


I noticed that a while ago the isSSA flag was removed from MIR, and this 
property is now computed. However, the deduction only checks the virtual 
registers (whether they are assigned to more than once). Now I have MIR 
tests which live after RA, so they only contain physical registers, so 
the parser determines they are in SSA form. These tests now fail because 
some of our passes can be called before and after SSA, so take the wrong 
Surely the isSSA computation is not comprehensive? It also doesn't take 
into account non-SSA tests which use virtual registers but happen to not 
assign twice to each.
Duplicate assignments imply not-SSA, but no duplicate (virtual reg) 
assignments doesn't imply SSA?
I noticed a Hexagon test (CodeGen/Hexagon/expand-condsets-rm-reg.mir) 
has a hack around this by inserting an additional COPY to a register, 
but that shouldn't be required. It means tests that are generated using 
llc might be failing because they are wrongly identified as SSA.


Verena Beckham

Senior Principal Software Engineer, Compilers

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