b812eec928
Currently, we compile range loops into for loops with the obvious initialization and update of the index variable. In this form, the prove pass can see that the body is dominated by an i < len condition, and findIndVar can detect that i is an induction variable and that 0 <= i < len. GOEXPERIMENT=preemptibleloops compiles range loops to OFORUNTIL and we're preparing to unconditionally switch to a variation of this for #24543. OFORUNTIL moves the increment and condition *after* the body, which makes the bounds on the index variable much less obvious. With OFORUNTIL, proving anything about the index variable requires understanding the phi that joins the index values at the top of the loop body block. This interferes with both prove's ability to see that i < len (this is true on both paths that enter the body, but from two different conditional checks) and with findIndVar's ability to detect the induction pattern. Fix this by teaching prove to detect that the index in the pattern constructed by OFORUNTIL is an induction variable and add both bounds to the facts table. Currently this is done separately from findIndVar because it depends on prove's factsTable, while findIndVar runs before visiting blocks and building the factsTable. Without any GOEXPERIMENT, this has no effect on std or cmd. However, with GOEXPERIMENT=preemptibleloops, this change becomes necessary to prove 90 conditions in std and cmd. Change-Id: Ic025d669f81b53426309da5a6e8010e5ccaf4f49 Reviewed-on: https://go-review.googlesource.com/102603 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org> |
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README.md
The Go Programming Language
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