0002354: RTE assertion for signed right shift is wrong

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Date Submitted

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0002354

Frama-C

Plug-in > RTE

public

2018-02-03 12:15

2018-11-30 10:07

Reporter

xleroy

Assigned To

signoles

Priority

normal

Severity

minor

Reproducibility

have not tried

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

Frama-C 16-Sulfur

Target Version

Fixed in Version

Frama-C 18-Argon

Summary

0002354: RTE assertion for signed right shift is wrong

Description

Consider: /*@ ensures \result == arg >> 2; */ int toto(int arg) { return arg >> 2; } "frama-c -wp -wp-rte" fails to verify this rather tautological spec because the RTE pass inserts the assertion /*@ assert rte: shift: 0 ≤ arg; */ I think this is an incorrect reading of the ISO C 90 and C 99 specs: right-shift of a signed integer expression that has a negative value is implementation-defined, but is not an undefined behavior. (See ISO C 99 section 6.5.7 item 5.) I would suggest that WP and RTE accept the spec above, so that I can actually reason about the right shift, probably by adding axioms specifying how right shifts of negative numbers work on the platforms I am (and everyone is) interested in.

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Notes
(0006664) signoles (manager) 2018-10-15 17:17	A new kernel option (-warn-right-shift-negative) has been introduced for controlling this behaviour. It is deactivated by default (while -warn-left-shift-negative is activated by default), hence no assertion is now emitted by default on your example. This option is supported by plug-ins Eva and RTE. The assertions generated by RTE impact plug-ins E-ACSL and WP.

Issue History
Date Modified	Username	Field	Change
2018-02-03 12:15	xleroy	New Issue
2018-02-03 12:15	xleroy	Status	new => assigned
2018-02-03 12:15	xleroy	Assigned To	=> signoles
2018-10-15 17:17	signoles	Note Added: 0006664
2018-10-15 17:17	signoles	Status	assigned => resolved
2018-10-15 17:17	signoles	Resolution	open => fixed
2018-11-30 10:07	signoles	Fixed in Version	=> Frama-C 18-Argon
2018-11-30 10:07	signoles	Status	resolved => closed