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cmd/compile: 64 bits shifts on arm get wrong results [1.16 backport] #48478

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gopherbot opened this issue Sep 20, 2021 · 3 comments
Closed

cmd/compile: 64 bits shifts on arm get wrong results [1.16 backport] #48478

gopherbot opened this issue Sep 20, 2021 · 3 comments
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CherryPickApproved Used during the release process for point releases FrozenDueToAge
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Go1.16.10

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@gopherbot
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@randall77 requested issue #48476 to be considered for backport to the next 1.16 minor release.

@gopherbot please open backport issues
@gopherbot gopherbot added the CherryPickCandidate Used during the release process for point releases label Sep 20, 2021
@gopherbot gopherbot mentioned this issue Sep 20, 2021
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@gopherbot gopherbot added this to the Go1.16.9 milestone Sep 20, 2021
@gopherbot
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Change https://golang.org/cl/351070 mentions this issue: cmd/compile: ensure constant shift amounts are in range for arm

@randall77
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This issue causes miscompilation. It's hard to trigger, but causes the compiler to generate wrong code if it does.

@heschi heschi added CherryPickApproved Used during the release process for point releases and removed CherryPickCandidate Used during the release process for point releases labels Oct 6, 2021
@mknyszek mknyszek modified the milestones: Go1.16.9, Go1.16.10 Oct 7, 2021
@dmitshur
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Closed by merging 1a6281d to release-branch.go1.16.

gopherbot pushed a commit that referenced this issue Oct 27, 2021
@randall77 @dmitshur
[release-branch.go1.16] cmd/compile: ensure constant shift amounts ar…
1a6281d 
…e in range for arm

Ensure constant shift amounts are in the range [0-31]. When shift amounts
are out of range, bad things happen. Shift amounts out of range occur
when lowering 64-bit shifts (we take an in-range shift s in [0-63] and
calculate s-32 and 32-s, both of which might be out of [0-31]).

The constant shift operations themselves still work, but their shift
amounts get copied unmolested to operations like ORshiftLL which use only
the low 5 bits. That changes an operation like <<100 which unconditionally
produces 0, to <<4, which doesn't.

Fixes #48478

Change-Id: I87363ef2b4ceaf3b2e316426064626efdfbb8ee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/350969
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
(cherry picked from commit eff27e8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/351070
Reviewed-by: Austin Clements <[email protected]>
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