nir: fix crash in loop unroll corner case
When an if nesting inside anouther if is optimised away we can end up with a loop terminator and following block that looks like this: if ssa_596 { block block_5: /* preds: block_4 */ vec1 32 ssa_601 = load_const (0xffffffff /* -nan */) break /* succs: block_8 */ } else { block block_6: /* preds: block_4 */ /* succs: block_7 */ } block block_7: /* preds: block_6 */ vec1 32 ssa_602 = phi block_6: ssa_552 vec1 32 ssa_603 = phi block_6: ssa_553 vec1 32 ssa_604 = iadd ssa_551, ssa_66 The problem is the phis. Loop unrolling expects the last block in the loop to be empty once we splice the instructions in the last block into the continue branch. The problem is we cant move phis so here we lower the phis to regs when preparing the loop for unrolling. As it could be possible to have multiple additional blocks/ifs following the terminator we just convert all phis at the top level of the loop body for simplicity. We also add some comments to loop_prepare_for_unroll() while we are here. Fixes: 51daccb2 "nir: add a loop unrolling pass" Reviewed-by:Jason Ekstrand <jason@jlekstrand.net> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105670 (cherry picked from commit 629ee690)
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