[LLVMbugs] [Bug 21903] Bad machine code: MBB exits via unconditional fall-through but its successor differs from its CFG successor

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21903

Dimitry Andric <dimitry at andric.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|WORKSFORME                  |---

--- Comment #3 from Dimitry Andric <dimitry at andric.com> ---
I got very similar crash report for the fontforge port [1], and this results in
a crash, even with trunk r224516.  Note that this only happens with assertions
enabled!

The resulting message is very similar, after BB dumps, it gives:

*** Bad machine code: MBB exits via unconditional fall-through but its
successor differs from its CFG successor! ***
- function:    SplineIsLinear
- basic block: BB#152 land.end.i573 (0x2b5254f8)

*** Bad machine code: MBB exits via unconditional fall-through but its
successor differs from its CFG successor! ***
- function:    SplineIsLinear
- basic block: BB#156 land.end11.i579 (0x2b525644)

*** Bad machine code: MBB exits via unconditional fall-through but its
successor differs from its CFG successor! ***
- function:    SplineIsLinear
- basic block: BB#159 if.then17.i588 (0x2b525790)

*** Bad machine code: MBB exits via unconditional fall-through but its
successor differs from its CFG successor! ***
- function:    SplineIsLinear
- basic block: BB#160 if.else19.i593 (0x2b5257ec)

*** Bad machine code: MBB exits via unconditional fall-through but its
successor differs from its CFG successor! ***
- function:    SplineIsLinear
- basic block: BB#162 if.then27.i600 (0x2b5258a4)

*** Bad machine code: Using an undefined physical register ***
- function:    SplineIsLinear
- basic block: BB#163 if.else32.i605 (0x2b525900)
- instruction: FNSTSW16r %AX<imp-def>, %FPSW<imp-use,kill>
- operand 1:   %FPSW<imp-use,kill>
fatal error: error in backend: Found 6 machine code errors.

Specifically, the "FNSTSW16r %AX<imp-def>, %FPSW<imp-use,kill>" is precisely
the same as in the Inkscape case.  I think this must be some sort of bug in the
(x87) floating point optimization.

I will add a reduced test case after this comment.

[1]
http://pb2.nyi.freebsd.org/data/head-i386-PR195480-default/2014-12-15_22h55m50s/logs/errors/fontforge-20120731.b_7.log

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