94cb129d51
The intersects() function assumes that inside each instruction values always die before they are defined, so that if the end of one range is the same instruction as the beginning of the next then they don't intersect. However, this isn't the case for values that become live at the beginning of a basic block, which become live *before* the first instruction, or instructions that die at the end of a basic block which die after the last instruction. For example, imagine that we have two values, A which is defined earlier in the block and B which is defined in the last instruction of the block and both die at the end of the basic block (e.g. are used in the next iteration of a loop). We would compute a range for A of, say, (10, 20) and for B of (20, 20) since each block's end_ip is the same as the ip of the last instruction, and RA would consider them to not interfere. There's a similar problem with values that become live at the beginning. The fix is to offset the block's start_ip and end_ip by one so that they don't correspond to any actual instruction. One way to think about this is that we're adding fake instructions at the beginning and end of a block where values become live & die. We could invert the order, so that values consumed by each instruction are considered dead at the end of the previous instruction, but then values that become dead at the beginning of the basic block would incorrectly have an empty live range, with a similar problem at the end of the basic block if we try to say that values are defined at the beginning of the next instruction. So the extra padding instructions are unavoidable. This fixes an accidental infinite loop in the shader for dEQP-VK.spirv_assembly.type.scalar.u32.switch_vert. Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4614>
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