r309422 | rnk | 2017-07-28 12:48:40 -0700 (Fri, 28 Jul 2017) | 25 lines
Fix conditional tail call branch folding when both edges are the same
The conditional tail call logic did the wrong thing when both
destinations of a conditional branch were the same:
BB#1: derived from LLVM BB %entry
Live Ins: %EFLAGS Predecessors according to CFG: BB#0 JE_1 <BB#5>, %EFLAGS<imp-use,kill> JMP_1 <BB#5>
BB#5: derived from LLVM BB %sw.epilog
Predecessors according to CFG: BB#1 TCRETURNdi64 <ga:@mergeable_conditional_tailcall>, 0, ...
We would fold the JE_1 to a TCRETURNdi64cc, and then remove our BB#5
successor. Then BB#5 would be deleted as it had no predecessors, leaving
a dangling "JMP_1 <BB#5>" reference behind to cause assertions later.
This patch checks that both conditional branch destinations are
different before doing the transform. The standard branch folding logic
is able to remove both the JMP_1 and the JE_1, and for my test case we
end up forming a better conditional tail call later.