69ff0ba798
Array accesses with index types smaller than the machine word size may involve a sign or zero extension of the index value before bounds checking. Currently, this defeats prove because the facts about the original index value don't flow through the sign/zero extension. This CL fixes this by looking back through value-preserving sign/zero extensions when adding facts via Update and, where appropriate, applying the same facts using the pre-extension value. This fix is enhanced by also looking back through value-preserving extensions within ft.isNonNegative to infer whether the extended value is known to be non-negative. Without this additional isNonNegative enhancement, this logic is rendered significantly less effective by the limitation discussed in the next paragraph. In Update, the application of facts to pre-extension values is limited to cases where the domain of the new fact is consistent with the type of the pre-extension value. There may be cases where this cross-domain passing of facts is valid, but distinguishing them from the invalid cases is difficult for me to reason about and to implement. Assessing which cases to allow requires details about the context and inferences behind the fact being applied which are not available within Update. Additional difficulty arises from the fact that the SSA does not curently differentiate extensions added by the compiler for indexing operations, extensions added by the compiler for implicit conversions, or explicit extensions from the source. Examples of some cases that would need to be filtered correctly for cross-domain facts: (1) A uint8 is zero-extended to int for indexing (a value-preserving zeroExt). When, if ever, can signed domain facts learned about the int be applied to the uint8? (2) An int8 is sign-extended to int16 (value-preserving) for an equality comparison. Equality comparison facts are currently always learned in both the signed and unsigned domains. When, if ever, can the unsigned facts learned about the int16, from the int16 != int16 comparison, be applied to the original int8? This is an alternative to CL 122695 and CL 174309. Compared to CL 122695, this CL differs in that the facts added about the pre-extension value will pass through the Update method, where additional inferences are processed (e.g. fence-post implications, see #29964). CL 174309 is limited to bounds checks, so is narrower in application, and makes the code harder to read. Fixes #26292. Fixes #29964. Fixes #15074 Removes 238 bounds checks from std/cmd. Change-Id: I1f87c32ee672bfb8be397b27eab7a4c2f304893f Reviewed-on: https://go-review.googlesource.com/c/go/+/174704 Run-TryBot: Zach Jones <zachj1@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Giovanni Bajo <rasky@develer.com> |
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