[LLVMdev] Unwanted push/pop on Cortex-M.

[Andrea Mucignat]

Hi,

I have this code:

void platform_putchar(int, char);
void out_char( char ch );

void out_char( char ch )
{
    platform_putchar (0, ch);
}

I'm compiling with the following clang invocation:

$ /usr/local/vendor/toolchains/llvm/3.3/armv7m/bin/armv7m-none-eabi-clang
-mcpu=cortex-m4 -mfloat-abi=soft -mthumb -nostdinc -ffreestanding
-ffunction-sections -fdata-sections -fno-exceptions -fno-rtti
-fomit-frame-pointer -momit-leaf-frame-pointer -nostdinc -v -Os -S -o a.s
a.c
vendor-clang version 3.3 (tags/RELEASE_33/final) (based on LLVM 3.3)
Target: armv7m-none--eabi
Thread model: posix
 "/usr/local/vendor-20130805-b8d59d2/toolchains/llvm/3.3/armv7m/bin/armv7m-none-eabi-clang"
-cc1 -triple thumbv7em-none--eabi -S -disable-free -main-file-name a.c
-mrelocation-model static -fmath-errno -mconstructor-aliases -target-abi
aapcs -target-cpu cortex-m4 -msoft-float -mfloat-abi soft -target-feature
+soft-float -target-feature +soft-float-abi -target-feature -neon
-target-linker-version 2.22 -momit-leaf-frame-pointer -v
-ffunction-sections -fdata-sections -coverage-file /tmp/a.s -nostdsysteminc
-nobuiltininc -resource-dir
/usr/local/vendor-20130805-b8d59d2/toolchains/llvm/3.3/armv7m/bin/../lib/clang/3.3
-Os -fno-dwarf-directory-asm -fdebug-compilation-dir /tmp -ferror-limit 19
-fmessage-length 207 -ffreestanding -mstackrealign -fno-rtti
-fno-signed-char -fobjc-runtime=gcc -fobjc-default-synthesize-properties
-fdiagnostics-show-option -fcolor-diagnostics -backend-option
-vectorize-loops -o a.s -x c a.c
clang -cc1 version 3.3 based upon LLVM 3.3 default target armv7m-none-eabi
#include "..." search starts here:
End of search list.

$ cat a.s
    .syntax unified
    .eabi_attribute 6, 10
    .eabi_attribute 9, 2
    .eabi_attribute 10, 5
    .fpu vfpv4
    .eabi_attribute 20, 1
    .eabi_attribute 21, 1
    .eabi_attribute 23, 3
    .eabi_attribute 24, 1
    .eabi_attribute 25, 1
    .eabi_attribute 44, 1
    .file    "a.c"
    .section    .text.out_char,"ax",%progbits
    .globl    out_char
    .align    2
    .type    out_char,%function
    .code    16
    .thumb_func
out_char:
    push.w    {r11, lr}
    mov    r1, r0
    movs    r0, #0
    bl    platform_putchar
    pop.w    {r11, pc}
.Ltmp0:
    .size    out_char, .Ltmp0-out_char

The one question I have is:
why can't this out_char function be compiled into:

out_char:
    mov r1, r0
    mov r0, #0
    bl platform_putchar
    bx lr

What's the clang/llvm module responsible for generating the prologue and
epilogue for this function?
I looked into CodeGen/PrologEpilogInserter, ARMFrameLowering.cpp
Thumb1FrameLowering.cpp but it's not 100% clear how the code gen decides
what the stack frame look like.

Here's the -emit-llvm output

target datalayout =
"e-p:32:32:32-i1:8:32-i8:8:32-i16:16:32-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:64:128-a0:0:32-n32-S64"
target triple = "thumbv7em-none--eabi"

; Function Attrs: nounwind optsize
define void @out_char(char)(i8 zeroext %ch) #0 {
entry:
  tail call void @llvm.dbg.value(metadata !{i8 %ch}, i64 0, metadata !10),
!dbg !11
  tail call void @platform_putchar(int, char)(i32 0, i8 zeroext %ch) #3,
!dbg !12
  ret void, !dbg !13
}

; Function Attrs: optsize
declare void @platform_putchar(int, char)(i32, i8 zeroext) #1

; Function Attrs: nounwind readnone
declare void @llvm.dbg.value(metadata, i64, metadata) #2

attributes #0 = { nounwind optsize "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-frame-pointer-elim-non-leaf"="false"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"unsafe-fp-math"="false" "use-soft-float"="true" }
attributes #1 = { optsize "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-frame-pointer-elim-non-leaf"="false"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"unsafe-fp-math"="false" "use-soft-float"="true" }
attributes #2 = { nounwind readnone }
attributes #3 = { nobuiltin nounwind optsize }

!llvm.dbg.cu = !{!0}

!0 = metadata !{i32 786449, metadata !1, i32 4, metadata !"nestlabs-clang
version 3.3 (tags/RELEASE_33/final) (based on LLVM 3.3)", i1 true, metadata
!"", i32 0, metadata !2, metadata !2, metadata !3, metadata !2, metadata
!2, metadata !""} ; [ DW_TAG_compile_unit ] [/tmp/a.c] [DW_LANG_C_plus_plus]
!1 = metadata !{metadata !"a.c", metadata !"/tmp"}
!2 = metadata !{i32 0}
!3 = metadata !{metadata !4}
!4 = metadata !{i32 786478, metadata !1, metadata !5, metadata !"out_char",
metadata !"out_char", metadata !"out_char(char)", i32 6, metadata !6, i1
false, i1 true, i32 0, i32 0, null, i32 256, i1 true, void (i8)*
@out_char(char), null, null, metadata !9, i32 7} ; [ DW_TAG_subprogram ]
[line 6] [def] [scope 7] [out_char]
!5 = metadata !{i32 786473, metadata !1}          ; [ DW_TAG_file_type ]
[/tmp/a.c]
!6 = metadata !{i32 786453, i32 0, i32 0, metadata !"", i32 0, i64 0, i64
0, i64 0, i32 0, null, metadata !7, i32 0, i32 0} ; [
DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset 0] [from ]
!7 = metadata !{null, metadata !8}
!8 = metadata !{i32 786468, null, null, metadata !"char", i32 0, i64 8, i64
8, i64 0, i32 0, i32 8} ; [ DW_TAG_base_type ] [char] [line 0, size 8,
align 8, offset 0, enc DW_ATE_unsigned_char]
!9 = metadata !{metadata !10}
!10 = metadata !{i32 786689, metadata !4, metadata !"ch", metadata !5, i32
16777222, metadata !8, i32 0, i32 0} ; [ DW_TAG_arg_variable ] [ch] [line 6]
!11 = metadata !{i32 6, i32 0, metadata !4, null}
!12 = metadata !{i32 8, i32 0, metadata !4, null}
!13 = metadata !{i32 9, i32 0, metadata !4, null}


Thanks
Andrea.
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