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I am attempting to compile a C++ library for a Tegra TK1. The library links to TBB, which I pulled using the package manager. During compilation I got the following error

/tmp/cc4iLbKz.s: Assembler messages:
/tmp/cc4iLbKz.s:9541: Error: thumb conditional instruction should be in IT block -- `strexeq r2,r3,[r4]'

A bit of googling and this question led me to try adding -mimplicit-it=thumb to CMAKE_CXX_FLAGS, but the compiler doesn't recognize it.

I am compiling on the tegra with kernal 3.10.40-grinch-21.3.4, and using gcc 4.8.4 compiler (thats what comes back when I type c++ -v)

I'm not sure what the initial error message means, though I think it has something to do with the TBB linked library rather than the source I'm compiling. The problem with the fix is also mysterious. Can anyone shed some light on this?

Community
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dmagree
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2 Answers2

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-mimplicit-it is an option to the assembler, not to the compiler. Thus, in the absence of specific assembler flags in your makefile (which you probably don't have, given that you don't appear to be using a separate assembler step), you'll need to use the -Wa option to the compiler to pass it through, i.e. -Wa,-mimplicit-it=thumb.

The source of the issue is almost certainly some inline assembly - possibly from a static inline in a header file if you're really only linking pre-built libraries - which contains conditionally-executed instructions (I'm going to guess its something like a cmpxchg implementation). Since your toolchain could well be configured to compile to the Thumb instruction set - which requires a preceding it (If-Then) instruction to set up conditional instructions - by default, another alternative might be to just compile with -marm (and/or remove -mthumb if appropriate) and sidestep the issue by not using Thumb at all.

Notlikethat
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  • I went with `-Wa,-mimplicit-it=thumb`, and that seems to have solved assembler error. Are there performance or other considerations that should inform my choice of using thumb or not? – dmagree Mar 15 '16 at 14:07
  • There's probably not a huge amount in it - there are still certain things you can do with an ARM instruction that need multiple Thumb instructions (even with Thumb-2); on the other hand Thumb code still tends to be smaller, so you make better use of the I-cache (although you can then potentially run into branch predictor corner cases more easily). You'd really need to do careful benchmarking on a case-by-case basis to work out whether you care. – Notlikethat Mar 15 '16 at 22:43
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Adding compiler option:

-wa

should solve the problem.

Bojan Mise
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