[PATCH] D91020: [X86] Unbind the ebx with GOT address in regcall calling convention

[LuoYuanke via Phabricator via llvm-commits]

LuoYuanke added inline comments.


================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:4120
   if (Subtarget.isPICStyleGOT()) {
     // ELF / PIC requires GOT in the EBX register before function calls via PLT
+    // GOT pointer (except regcall).
----------------
To be conservative, do you need to check if the argument number exceed 4 (>=5)?


================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:4136
 
       // Note: The actual moving to ECX is done further down.
       GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee);
----------------
xiangzhangllvm wrote:
> LuoYuanke wrote:
> > Would you add a test case for tail call? Is there any conflict to ECX?
> I check here local before, For X86, tail call will not work with regcall, For X86_64 it will work, but X86_64 don't fix ebx with GOT point.
> please check the following case, it will not generate jump  for 2nd command.
> 
> ```
>   1 ; llc -mtriple=x86_64-unknown-linux-gnu -relocation-model=pic
>   2 ; llc -mtriple=i386-unknown-linux-gnu -relocation-model=pic  <-tailcallopt>
>   3
>   4 declare x86_regcallcc void @regcall_not_lazy(i32 %a0, i32 %b0)
>   5
>   6 define void @tail_call_regcall() nounwind {
>   7   tail call x86_regcallcc void @regcall_not_lazy(i32 0, i32 1)
>   8   ret void
>   9 }
> ```
It seems compiler generate jmp instruction only when the argument number is less or equal to 2 and without pic relocation model.
```
; llc -mtriple=x86_64-unknown-linux-gnu
; llc -mtriple=i386-unknown-linux-gnu

@foo6 = external global void (i32 %0, i32 %1, i32 %2, i32 %3, i32 %4, i32 %5)*

define void @tail_call_regcall6(i32 %a, i32 %b, i32 %c, i32 %d, i32 %e, ...) nounwind {
  %t0 = alloca i32, align 128
  %t1 = load void (i32, i32, i32, i32, i32, i32)*, void (i32, i32, i32, i32, i32, i32)** @foo6, align 4
  tail call x86_regcallcc void %t1(i32 0, i32 1, i32 2, i32 3, i32 4, i32 5) nounwind
  ret void
}

@foo5 = external global void (i32 %0, i32 %1, i32 %2, i32 %3, i32 %4)*

define void @tail_call_regcall5(i32 %a, i32 %b, i32 %c, i32 %d, i32 %e) nounwind {
  %t1 = load void (i32, i32, i32, i32, i32)*, void (i32, i32, i32, i32, i32)** @foo5, align 4
  ; tail call x86_regcallcc void %t1(i32 0, i32 1, i32 2, i32 3, i32 4) nounwind
  tail call x86_regcallcc void %t1(i32 %a, i32 %b, i32 %c, i32 %d, i32 %e) nounwind
  ret void
}

@foo4 = external global void (i32 %0, i32 %1, i32 %2, i32 %3)*

define void @tail_call_regcall4(i32 %a, i32 %b, i32 %c, i32 %d) nounwind {
  %t1 = load void (i32, i32, i32, i32)*, void (i32, i32, i32, i32)** @foo4, align 4
  ; tail call x86_regcallcc void %t1(i32 0, i32 1, i32 2, i32 3, i32 4) nounwind
  tail call x86_regcallcc void %t1(i32 %a, i32 %b, i32 %c, i32 %d) nounwind
  ret void
}

@foo3 = external global void (i32 %0, i32 %1, i32 %2)*

define void @tail_call_regcall3(i32 %a, i32 %b) nounwind {
  %t1 = load void (i32, i32, i32)*, void (i32, i32, i32)** @foo3, align 4
  tail call x86_regcallcc void %t1(i32 0, i32 1, i32 2) nounwind
  ret void
}

@foo2 = external global void (i32 %0, i32 %1)*

define void @tail_call_regcall2(i32 %a, i32 %b) nounwind {
  %t1 = load void (i32, i32)*, void (i32, i32)** @foo2, align 4
  tail call x86_regcallcc void %t1(i32 0, i32 1) nounwind
  ; tail call x86_regcallcc void %t1(i32 %a, i32 %b) nounwind
  ret void
}
```


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D91020/new/

https://reviews.llvm.org/D91020