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For C++ exception handling, when we write a call site table, we must avoid emitting 0-value offsets for landing pads unless the call site has no landing pad. However, 0 can be a real offset from the start of the FDE if the FDE corresponds to a function fragment that starts with a landing pad. In such cases, we used to emit a trap instruction at the start of the fragment to guarantee non-zero LP offset. To avoid emitting unnecessary trap instructions, we can instead set LPStart to an offset from the FDE. If we emit it as [FDEStart - 1], then all real offsets from LPStart in FDE become non-negative.