Compilation fails with backend ran out of registers about popcorn-compiler HOT 1 OPEN

More background

The rematerializing code is built to cheaply reproduce a small number of values for a single instruction. Since almost all machine instructions only use a handful of values, reproducing this small set of values right before the instruction doesn't normally cause issues. The rematerialization code creates small live ranges containing the remat instruction and its use, and prevents if from being evicted or spilled again.

The stackmap intrinsic violates the assumption of a few small values and can have a large number of live values that are rematerialized just for inclusion in the stackmap. This depletes the pool of available registers, causing this error. Note that this issue isn't encountered for call instructions which take a large number of values -- they are custom lowered so that all argument registers are filled and any remaining arguments are spilled to the stack appropriately.

See [1] for more information.

Quick and dirty workaround

The root cause of this issue is a large number of live values across stackmaps. Optimizations exacerbate this problem (as they often trade time for space or increased code size); in particular, inlining seems to cause this in some instances. Compiling with -fno-inline may work around the issue. If not, turning the optimization level to -O0 may also eliminate the issue.

[1] "LLVM Register Allocation" https://www.slideshare.net/chimerawang/llvm-register-allocation-59885569

from popcorn-compiler.