This assertion is observed at least on linux-i586/amd64 with both latest jdk6 (1.6.0_10-ea-b09) and jdk7 (1.7.0-ea-b24):
# Internal Error (/BUILD_AREA/jdk7/hotspot/src/share/vm/opto/superword.cpp:437), pid=15100, tid=2994375600
# Error: assert(in_bb(n),"must be in block")
It occurs only in -Xcomp mode.
Testcase and hs_err file are attached.
Attached a testcase for solaris-i586/amd64.
The assertion occurs only with C2.