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The conceptual error was the assumption that not being under varying control flow implied that the mask was all on; this is not the case if some of the instances have executed a return earlier in the function's execution. The error in practice would be that the mask would be assumed to be all-on for things like memory writes, so there would be unintended side-effects for the instances that had returned.
============================== Intel(r) SPMD Program Compiler ============================== Welcome to the Intel(r) SPMD Program Compiler (ispc)! ispc is a new compiler for "single program, multiple data" (SPMD) programs. Under the SPMD model, the programmer writes a program that mostly appears to be a regular serial program, though the execution model is actually that a number of program instances execute in parallel on the hardware. ispc compiles a C-based SPMD programming language to run on the SIMD units of CPUs; it frequently provides a a 3x or more speedup on CPUs with 4-wide SSE units, without any of the difficulty of writing intrinsics code. ispc is an open source compiler under the BSD license; see the file LICENSE.txt. ispc supports Windows, Mac, and Linux, with both x86 and x86-64 targets. It currently supports the SSE2, SSE4, and AVX instruction sets. For more information and examples, as well as a wiki and the bug database, see the ispc distribution site, http://ispc.github.com.