[llvm-dev] Invalid instruction generated on armV4

[Frédéric Richez via llvm-dev]

I’m tying to run a rust application on armv4 architecture (arm720tdmi).
Rust is using llvm to generate native code.
Some programs are running well on the target but for more complex applications I receive a Illegal instruction :
gdb disassembling the core dump file gives :

   0x401e41dc <+0>:	push	{r11, lr}
   0x401e41e0 <+4>:	mov	r11, sp
   0x401e41e4 <+8>:	sub	sp, sp, #8
   0x401e41e8 <+12>:	ldr	r0, [r0]
   0x401e41ec <+16>:	str	r0, [sp, #4]
   0x401e41f0 <+20>:	mov	r0, r1
   0x401e41f4 <+24>:	ldr	r1, [sp, #4]
=> 0x401e41f8 <+28>:	blx	r1
   0x401e41fc <+32>:	and	r0, r0, #1
   0x401e4200 <+36>:	and	r0, r0, #1
   0x401e4204 <+40>:	mov	sp, r11
   0x401e4208 <+44>:	pop	{r11, lr}
   0x401e420c <+48>:	bx	lr
blx instruction is available from ARM architecture v5 and above…

After lot of investigations I found where llvm is generating this instruction :
https://github.com/rust-lang/llvm/blob/168f91ce5cbf8933e47f339911f0f46a48714852/lib/Target/ARM/ARMFastISel.cpp#L2395 <https://github.com/rust-lang/llvm/blob/168f91ce5cbf8933e47f339911f0f46a48714852/lib/Target/ARM/ARMFastISel.cpp#L2395>
UseReg variable is true causing ARMSelectCallOp function to set a BLX instruction that is not supported on armv4 :-(

Could you help me to solve this ? 

Many thanks

Frédéric.

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