[GHC] #13425: Semantically-equivalent expressions evaluating at different values with -O1

[GHC]

#13425: Semantically-equivalent expressions evaluating at different values with -O1
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        Reporter:  rbasso            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3359
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"caf94b062a0e37ffa7048e51447fc9486b658917/ghc"
 caf94b06/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="caf94b062a0e37ffa7048e51447fc9486b658917"
 x86 nativeGen: Fix test with mask in range [128,255] (#13425)

 My commit bdb0c43c7 optimized the encoding of instructions to test
 tag bits, but it did not always set exactly the same condition codes
 since the testb instruction does a single-byte comparison, rather
 than a full-word comparison.

 It would be correct to optimize the expression `x .&. 128 > 0` to
 the sequence

     testb $128, %al
     seta %al         ; note: 'a' for unsigned comparison,
                      ; not 'g' for signed comparison

 but the pretty-printer is not the right place to make this kind of
 context-sensitive optimization.

 Test Plan: harbormaster

 Reviewers: trofi, austin, bgamari, dfeuer

 Reviewed By: trofi, dfeuer

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D3359
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13425#comment:6>
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