Noinline, no-opt method loses call to other method -O3

LLVM Project
1

Godbolt link: Compiler Explorer

Got a fairly simple function (BADCALLER): that is noinline, optnone, it calls BADCALLEE, however neither the call nor the inlined code is present in the result:

; ModuleID = 'Type.get_Methodsd__6-f73a26d932fbf794da5cc508811d5f14'
target triple = "x86_64-pc-windows-msvc"

%typeobj = type { %rstObject, ptr, ptr, i32, ptr, i32, i32 }
%rstObject = type { ptr }

; Function Attrs: noinline optnone
define void @"BADCALLER"(i32 %x0, ptr %x1, i64 %x2, i64 %x3) #5 {
  %x5 = urem i32 %x0, 11
  switch i32 %x5, label %x6 [
    i32 6, label %x28
  ]

x6:                                                ; preds = %x55, %x45, %x35, %x16, %x4
  call void @__abstract()
  unreachable

x28:                                               ; preds = %x29, %x26
  musttail call void @"BADCALLEE"(i32 %x0, ptr %x1, i64 %x2, i64 %x3)
  ret void
}

define i8 @"$u8"(i32 %x0, ptr nocapture readnone %x1, ptr %x2) unnamed_addr {
  ret i8 0
}

; Function Attrs: uwtable
declare void @__abstract() local_unnamed_addr #4


; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(argmem: read)
define ptr @"BADCALLEE"(i32 %x0, ptr nocapture readnone %x1, ptr nocapture readonly %x2) unnamed_addr #7 {
  %x4 = getelementptr %typeobj, ptr %x2, i64 0, i32 1
  %x5 = load ptr, ptr %x4, align 8
  ret ptr %x5
}


attributes #4 = { uwtable }
attributes #5 = { noinline optnone "thunk"="true" }
attributes #7 = { mustprogress nofree norecurse nosync nounwind willreturn memory(argmem: read) }

Result:

        .def    BADCALLER;
        .scl    3;
        .type   32;
        .endef
        .p2align        4, 0x90                         # -- Begin function BADCALLER
BADCALLER:                              # @BADCALLER
.seh_proc BADCALLER
# %bb.0:
        sub     rsp, 40
        .seh_stackalloc 40
        .seh_endprologue
        imul    eax, ecx, -1171354717
        add     eax, -1561806290
        sub     eax, 390451572
        jb      .LBB0_2
        jmp     .LBB0_1
.LBB0_1:                                # %x6
        call    __abstract
.LBB0_2:                                # %x28
        nop
        add     rsp, 40
        ret
        .seh_endproc

Note how it does call __abstract but not BADCALLEE.

Can someone tell me what I’m doing wrong?

(Cmdline -O3 -filetype=asm)

2

Your function has internal linkage but no uses. It is simply deleted.

3

Fixed (doesn’t change the effect).

What seems to happen is that because the result of the musttail call isn’t used (it’s a thunk), and the function has no side effects, it’s completely eliminated.

Yep:

  %a = musttail call i64 @"BADCALLEE"(i32 %x0, ptr %x1, i64 %x2, i64 %x3)
  ret i64 %a

(And changing the result of BADCALLER ofc) fixes it.

4

Just removing memory(argmem: read) from the callee attributes is enough to make it preserving the jmp

1 Like