rwbarton authored Mar 23, 2017 and Ben Gamari committed Mar 24, 2017

My commit bdb0c43c 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