Use armv6t2 instead of armv6

[TimNN]

This is required to correctly build compiler-rt of rust-lang/rust with LLVM 4.0.

I don't know what exactly the consequences of this change are, however in general armv6t2 seems to be widely supported. However note that the raspberry pi 1, for example, apparently does not support Thumb 2. See also the Wikipedia List of ARM Microarchitectures.

cc @nagisa: You have done some research into this as well, I believe.

cc rust-lang/rust#40123

[nagisa]

Is there a way to disable both the thumbs? If not, then this seems like an obvious sol'n to me.

[TimNN]

Is there a way to disable both the thumbs?

I have no idea. Gcc accepts armv6 armv6-m armv6j armv6k armv6kz armv6s-m armv6t2 armv6z armv6zk for -march however I think they all imply at least Thumb1, with some implying only a subset of Thumb2.

[alexcrichton]

This comment seems quite relevent perhaps?

Currently arm-unknown-linux-gnu is sort of our "maxially compatible ARM" target, so if we're slicing out platforms (esp rpi) we may want to try to work around this another way unfortunately :(

[nagisa]

So I tried to use a number of options to see if any of them would remove the define for thumb, sadly none of them except redefining the environment variable manually helped:

  -marm                       Generate code in 32 bit ARM state. (does not disable the THUMB define, though)
  -mno-thumb                  (CLANG ONLY: does not disable the thumb define either)
  -march=*                    Some thumb gets defined for all of them
  -D__ARM_ARCH_ISA_THUMB=0    (WORKS) Redefine manually to something else.

This is how I was testing it:

arm-none-eabi-gcc -dM -E -xc {PARAMS} /dev/null | grep THUMB

So I suppose simply adding this define for our own compiler-rt compilation is the way to go, leaving gcc-rs alone.

[nagisa]

A small aside, I read a recommendation in multiple locations that if thumb2 is not available, -marm should be used, because thumb1 by itself is terrible and everything.

[TimNN]

Using a different fix for now (-D__ARM_ARCH_ISA_THUMB=0 ).

[MineFuehrer]

@TimNN
Stop running with scissors man! The only practical user of this target is the Rpi(A,B,zero). Learn the basics before proposing any more "fixes" like this.

[MineFuehrer]

https://archive.is/ACutm#selection-22847.0-22865.15
https://archive.is/o2yuQ