[llvm-dev] [ARM/Thumb] Make a function in arm while in Thumb triple

[Evgeny Astigeevich via llvm-dev]

Hi Dimitri,

As I know the following ACLE attributes are not supported in Clang/LLVM at the moment:

__attribute__((target("arm"))) attribute
__attribute__((target("thumb"))) attribute

But there is a workaround:

__attribute__((target("no-thumb-mode")))
__attribute__((target("thumb-mode")))

The attributes are propagated to a function declaration in IR by the frontend and the backend generated the code according to them:

$ cat test.c
    __attribute__((target("no-thumb-mode"))) int foo(void)
    {
        return 0;
    }
    int main(void)
    {
        return foo();
    }

$ clang -nostdlib -target armv4t-none-eabi -mthumb -c test.c -emit-llvm -S -o -
; ModuleID = 'test.c'
source_filename = "test.c"
target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "thumbv4t-none--eabi"

; Function Attrs: noinline nounwind
define i32 @foo() #0 {
  ret i32 0
}

; Function Attrs: noinline nounwind
define i32 @main() #1 {
  %1 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  %2 = call i32 @foo()
 ret i32 %2
}

attributes #0 = { noinline nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="arm7tdmi" "target-features"="+soft-float-abi,+strict-align,-thumb-mode" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { noinline nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="arm7tdmi" "target-features"="+strict-align" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0, !1}
!llvm.ident = !{!2}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 1, !"min_enum_size", i32 4}
!2 = !{!"clang version 4.0.1 (branches/release_40 299654)"}

$ clang -nostdlib -target armv4t-none-eabi -mthumb -c test.c -S -o -
        .text
        .syntax unified
        .eabi_attribute 67, "2.09"      @ Tag_conformance
        .cpu    arm7tdmi
        .eabi_attribute 6, 2    @ Tag_CPU_arch
        .eabi_attribute 8, 1    @ Tag_ARM_ISA_use
        .eabi_attribute 9, 1    @ Tag_THUMB_ISA_use
        .eabi_attribute 17, 1   @ Tag_ABI_PCS_GOT_use
        .eabi_attribute 20, 1   @ Tag_ABI_FP_denormal
        .eabi_attribute 21, 1   @ Tag_ABI_FP_exceptions
        .eabi_attribute 23, 3   @ Tag_ABI_FP_number_model
        .eabi_attribute 34, 0   @ Tag_CPU_unaligned_access
        .eabi_attribute 24, 1   @ Tag_ABI_align_needed
        .eabi_attribute 25, 1   @ Tag_ABI_align_preserved
        .eabi_attribute 38, 1   @ Tag_ABI_FP_16bit_format
        .eabi_attribute 18, 4   @ Tag_ABI_PCS_wchar_t
        .eabi_attribute 26, 2   @ Tag_ABI_enum_size
        .eabi_attribute 14, 0   @ Tag_ABI_PCS_R9_use
        .file   "test.c"
'+soft-float-abi' is not a recognized feature for this target (ignoring feature)
'+soft-float-abi' is not a recognized feature for this target (ignoring feature)
        .globl  foo
        .p2align        2
        .type   foo,%function
        .code   32                      @ @foo
foo:
        .fnstart
@ BB#0:
        mov     r0, #0
        bx      lr
.Lfunc_end0:
        .size   foo, .Lfunc_end0-foo
        .cantunwind
        .fnend

        .globl  main
        .p2align        1
        .type   main,%function
        .code   16                      @ @main
        .thumb_func
main:
        .fnstart
@ BB#0:
        .save   {r7, lr}
        push    {r7, lr}
        .setfp  r7, sp
        add     r7, sp, #0
        .pad    #8
        sub     sp, #8
        movs    r0, #0
        str     r0, [sp, #4]
        bl      foo
        add     sp, #8
        pop     {r7}
        pop     {r1}
        mov     lr, r1
        bx      lr
.Lfunc_end1:
        .size   main, .Lfunc_end1-main
        .cantunwind
        .fnend


        .ident  "clang version 4.0.1 (branches/release_40 299654)"
        .section        ".note.GNU-stack","",%progbits
        .eabi_attribute 30, 5   @ Tag_ABI_optimization_goals


Thanks,
Evgeny Astigeevich
Senior Compiler Engineer
Compilation Tools
ARM


From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Dimitri Tavan via llvm-dev
Sent: Tuesday, May 02, 2017 1:12 PM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] [ARM/Thumb] Make a function in arm while in Thumb triple

Hi,
I wanted to know if it was possible to force ARM backend to compile a function in ARM while the rest is in Thumb mode.
I tried the attributes which is used in GCC but it doesn't work.

Here is what I tried:
https://pastebin.com/jCr5LPUY

Thanks in advance,
Uvekilledkenny
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