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Originally, when this code was part of 6l/8l, every new Prog was constructed starting with zprg, which set back=2, and then this code walked over the list setting back=1 for backward branches, back=0 otherwise. The initial back=2 setting was used to identify forward branches (the branched-to instruction had back == 2 since it hadn't yet been set to 0 or 1). When the code was extracted into liblink and linked directly with 6a/6g/8a/8g, those programs created the Prog struct and did not set back=2, breaking this backward branch detection. No one noticed, because the next loop recomputes the information. The only requirement for the next loop is that p->back == 0 or 1 for each of the Progs in the list. The initialization of the zprg with back=2 would cause problems in this second loop, for the few liblink-internally-generated instructions that are created by copying zprg, except that the first loop was making sure that back == 0 or 1. The first loop's manipulation of p->back can thus be deleted, provided we also delete the zprg.back = 2 initializations. This is awful and my fault. I apologize. While we're here, remove the .scale = 1 from the zprg init too. Anything that sets up a scaled index should set the scale itself. (And mostly those come from outside liblink anyway.) Tested by checking that all generated code is bit-for-bit identical to before this CL. Change-Id: I7f6e0b33ce9ccd5b7dc25e0f00429fedd0957c8c Reviewed-on: https://go-review.googlesource.com/3574 Reviewed-by: Austin Clements <austin@google.com> |
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