`__attribute__((nocf_check))` on function pointer doesn't add `notrack` prefix with -O1+

[sroettger]

I'm trying to use the nocf_check attribute on a function pointer by reproducing this testcase:
https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/X86/nocf_check.ll#L7

void __attribute__((nocf_check)) NoCfCheckFunc(void) {}

typedef void(*FuncPointer)(void);
void NoCfCheckCall(FuncPointer f) {
  __attribute__((nocf_check)) FuncPointer p = (__attribute__((nocf_check)) FuncPointer) (f);
  (*p)();
}

Compiling this with -fcf-protection=branch my expectation is that the call to p should have a notrack prefix.
This is the case, but only if I add -O0. Any higher optimization level results in a regular branch without the notrack.

https://godbolt.org/z/ocnz1qhbW

cc @MaskRay

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: None (sroettger)

I'm trying to use the `nocf_check` attribute on a function pointer by reproducing this testcase: https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/X86/nocf_check.ll#L7

void __attribute__((nocf_check)) NoCfCheckFunc(void) {}

typedef void(*FuncPointer)(void);
void NoCfCheckCall(FuncPointer f) {
  __attribute__((nocf_check)) FuncPointer p = (__attribute__((nocf_check)) FuncPointer) (f);
  (*p)();
}

Compiling this with -fcf-protection=branch my expectation is that the call to p should have a notrack prefix.
This is the case, but only if I add -O0. Any higher optimization level results in a regular branch without the notrack.

https://godbolt.org/z/ocnz1qhbW

cc @MaskRay

[MaskRay]

__attribute__((nocf_check)) is rarely used (even within the Linux kernel).

reviews.llvm.org/D41879 implemented NOTRACK variants for
X86ISD::CALL and ISD::BRIND but not for TCRETURN. Given that there are
so many tail call variants (e.g. conditional tailcall
reviews.llvm.org/D29856), let's just disable tailcall.
#131487