forked from OSchip/llvm-project
c0b268f90c
In the patch rL329547, we have lifted the over-restrictive limitation on collected range checks, allowing to work with range checks with the end of their range not being provably non-negative. However it appeared that the non-negativity of this value was assumed in the utility function `ClampedSubtract`. In particular, its reasoning is based on the fact that `0 <= SINT_MAX - X`, which is not true if `X` is negative. The function `ClampedSubtract` is only called twice, once with `X = 0` (which is OK) and the second time with `X = IRC.getEnd()`, where we may now see the problem if the end is actually a negative value. In this case, we may sometimes miscompile. This patch is the conservative fix of the miscompile problem. Rather than rejecting non-provably non-negative `getEnd()` values, we will check it for non-negativity in runtime. For this, we use function `smax(smin(X, 0), -1) + 1` that is equal to `1` if `X` is non-negative and is equal to 0 if `X` is negative. If we multiply `Begin, End` of safe iteration space by this function calculated for `X = IRC.getEnd()`, we will get the original `[Begin, End)` if `IRC.getEnd()` was non-negative (and, thus, `ClampedSubtract` worked correctly) and the empty range `[0, 0)` in case if ` IRC.getEnd()` was negative. So we in fact prohibit execution of the main loop if at least one of range checks was made against a negative value (and we figured it out in runtime). It is still better than what we have before (non-negativity had to be proved in compile time) and prevents us from miscompile, however it is sometiles too restrictive for unsigned range checks against a negative value (which in fact can be eliminated). Once we re-implement `ClampedSubtract` in a way that it handles negative `X` correctly, this limitation can be lifted, too. Differential Revision: https://reviews.llvm.org/D46860 Reviewed By: samparker llvm-svn: 332809 |
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| .. | ||
| add-metadata-pre-post-loops.ll | ||
| bad-loop-structure.ll | ||
| bad_expander.ll | ||
| bug-loop-varying-upper-limit.ll | ||
| bug-mismatched-types.ll | ||
| clamp.ll | ||
| conjunctive-checks.ll | ||
| correct-loop-info.ll | ||
| decrementing-loop.ll | ||
| empty_ranges.ll | ||
| eq_ne.ll | ||
| low-becount.ll | ||
| multiple-access-no-preloop.ll | ||
| non_known_positive_end.ll | ||
| not-likely-taken.ll | ||
| only-lower-check.ll | ||
| only-upper-check.ll | ||
| optimistic_scev.ll | ||
| pre_post_loops.ll | ||
| range_intersect_miscompile.ll | ||
| ranges_of_different_types.ll | ||
| rc-negative-bound.ll | ||
| single-access-no-preloop.ll | ||
| single-access-with-preloop.ll | ||
| skip-profitability-checks.ll | ||
| stride_more_than_1.ll | ||
| unhandled.ll | ||
| unsigned_comparisons_ugt.ll | ||
| unsigned_comparisons_ult.ll | ||
| variable-loop-bounds.ll | ||
| with-parent-loops.ll | ||