There are six hits for this in stdlib, two of them in optimize.tl itself. The situation is like: label1 (instruction ...) (jmp label3) label2 (instruction ...) label3 where (instruction ...) is identical in both places. label1 and label2 are functionally identical blocks, which means that the pattern can be rewritten as: label1 label2 (instruction ...) label3 When the label1 path is taken it's faster due to the elimination of the jmp, and code size is reduced by two instructions. This pattern may possibly the result of an imperfection in the design of the basic-blocks method merge-jump-thunks. The label1 and label2 blocks are functionally identical. But merge-jump-thunks looks strictly for blocks that end in a jmp instruction. It's possible that there was a jmp instruction and the end of the label2 block, which got eliminated before merge-jump-thunks, which is done late, just before late-peephole. * stdlib/optimize.tl (basic-blocks late-peephole): New rule for the above pattern.