Representation -------------- Represent ALU32 recurrences as: w0 += step -> (zext32 (linear r0 step)) Extend simplify() rules to maintain the above form for latch expressions. E.g. for a post-condition loop such as: w0 = 0; l: w0 += 1; if w0 != 3 goto l; exit; SCEV at header: (zext32 (linear r0 1)) Initial SCEV at latch: (zext32 (+ (zext32 (linear r0 1)) 1)) Simplified SCEV at latch: (zext32 (+ (zext32 (linear r0 1)) 1)) -> (zext32 (+ (linear r0 1) 1)) -> (zext32 (linear (+ r0 1) 1)) Iterations count computation ---------------------------- - Extract the main iteration-count logic from compute_max_iters() into compute_backedge_taken_count(). Reuse its 64-bit arithmetic after sign/zero extending the latch operands. - Reject the computed count if the predicted exit value falls outside signed/unsigned 32-bit range, e.g.: w0 = 0x7ffffffe; l: w0 += 1; if w0 s<= 0x7fffffff goto l; The 64-bit computation predicts an exit at S32_MAX + 1, but the 32-bit value wraps to S32_MIN and the comparison remains true. - Handle mixed 64-bit/32-bit comparisons/recurrences only when initial bounds and step fit in 32-bit ranges, e.g.: w0 = 0; r1 = 0x100000003 ll; l: w0 += 1; if r0 != r1 goto l; Truncating the bound to 32 bits would predict three header visits. However, `w0 += 1` keeps r0 within [0, U32_MAX], so it can never be equal to the 64-bit bound. Widened bounds computation -------------------------- - Update linear_bounds() to truncate the initially computed 64-bit range to 32 bits, then zero-extend it to obtain the 64-bit range of iteration values. Step computation needs to handle: - unsigned 32-bit wraparound; - entry and zero-extended iteration values disagreeing on base/step. - Union both ranges with the corresponding register ranges at entry, since the entry value need not be zero-extended. - Extend `struct bounds` to keep both {32,64}-bit ranges and pass both to bpf_set_reg_range(). A 64-bit range alone loses useful 32-bit information, e.g.: entry = 0, step = -1, two header visits r64: [0, U32_MAX] r32: [-1, 0] Signed-off-by: Eduard Zingerman <eddyz87@gmail.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://patch.msgid.link/20261020-scev-32bit-v1-1-838f34f9f0cd@gmail.com