[llvm-dev] llc generating code that writes below the stack pointer on darwin/x86-64

[Jonas Maebe via llvm-dev]

Hi,

Is there something wrong with my inline assembly below?

***
target datalayout = 
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.5"

define void @"\01_SYSTEM_$$_SETMXCSR$LONGWORD"(i32 %p.w) nobuiltin {
; [71] procedure SetMXCSR(w : dword);
         %tmp.1 = alloca i32, align 4
; Var w located at %tmp.1
; [72] begin
         store i32 %p.w, i32* %tmp.1, align 4
; [73] defaultmxcsr:=w;
         %reg.1_16 = load i32, i32* %tmp.1, align 4
         store i32 %reg.1_16, i32* @"\01_TC_$SYSTEM_$$_DEFAULTMXCSR", 
align 4
; [75] ldmxcsr w
         call void asm sideeffect "# 
[math.inc]\0A\09ldmxcsr\09$0\0A","=*m,~{memory},~{fpsr},~{flags},~{rax},~{rcx},~{rdx},~{rsi},~{rdi},~{r8},~{r9},~{r10},~{r11}"(i32* 
%tmp.1)
; [77] end;
         br label %Lj1135
Lj1135:
         ret void
}

@"\01_TC_$SYSTEM_$$_DEFAULTMXCSR" = global i32 6400, align 4
***

I know the clobber list is overly conservative, but it's automatically 
generated by our compiler, which does not have the necessary information 
to reduce it.

The resulting assembly generated by llc -O2 is:

***
         .section        __TEXT,__text,regular,pure_instructions
         .macosx_version_min 10, 5
         .globl  _SYSTEM_$$_SETMXCSR$LONGWORD
         .p2align        4, 0x90
_SYSTEM_$$_SETMXCSR$LONGWORD:           ## 
@"\01_SYSTEM_$$_SETMXCSR$LONGWORD"
         .cfi_startproc
## BB#0:
         movl    %edi, -4(%rsp)
         movl    %edi, _TC_$SYSTEM_$$_DEFAULTMXCSR(%rip)
         ## InlineAsm Start
         ## [math.inc]
         ldmxcsr -4(%rsp)
         ##  CPU ATHLON64

         ## InlineAsm End
         retq
         .cfi_endproc

         .section        __DATA,__data
         .globl  _TC_$SYSTEM_$$_DEFAULTMXCSR ## 
@"\01_TC_$SYSTEM_$$_DEFAULTMXCSR"
         .p2align        2
_TC_$SYSTEM_$$_DEFAULTMXCSR:
         .long   6400                    ## 0x1900


.subsections_via_symbols
***

This code use a negative stack offset, which is forbidden on x86-64 
(there's no red zone). This happens with llc from llvm 3.7, 3.8 and 3.9.

Thanks,


Jonas