[MachineOutliner] Don't outline from functions with a section marking.

[efriedma]

Summary

The program might have unusual expectations for functions with section markings; for example, the Linux kernel's build system prints an error if it finds references from .text to .init.data.

I'm not sure this is something we actually want to make any guarantees about (there isn't any explicit rule that would disallow outlining in this case), but we might want to be conservative anyway.

[paquette]

LGTM.

(If we want to outline from these, we can probably try performing a separate outlining pass for each section. That way we can enforce that functions outlined from a specific section all come from the same section, and also end up in that *same* section.)

llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.cpp

 bool AArch64InstrInfo::isFunctionSafeToOutlineFrom(
     MachineFunction &MF, bool OutlineFromLinkOnceODRs) const {
   const Function &F = MF.getFunction();
 
   // Can F be deduplicated by the linker? If it can, don't outline from it.
   if (!OutlineFromLinkOnceODRs && F.hasLinkOnceODRLinkage())
     return false;
 
+  // Don't outline from functions with section markings; the program could
+  // expect that all the code is in the named section.
+  // FIXME: Allow outlining from multiple functions with the same section
+  // marking.
+  if (F.hasSection())
+    return false;
+
   // Outlining from functions with redzones is unsafe since the outliner may
   // modify the stack. Check if hasRedZone is true or unknown; if yes, don't
   // outline from it.
   AArch64FunctionInfo *AFI = MF.getInfo<AArch64FunctionInfo>();
   if (!AFI || AFI->hasRedZone().getValueOr(true))
     return false;
 
   // It's safe to outline from MF.

llvm/trunk/test/CodeGen/AArch64/machine-outliner.ll

define linkonce_odr void @fish() #0 {
   %4 = alloca i32, align 4
   store i32 0, i32* %1, align 4
   store i32 1, i32* %2, align 4
   store i32 2, i32* %3, align 4
   store i32 3, i32* %4, align 4
   ret void
 }
 
+define void @turtle() section "TURTLE,turtle" {
+  ; CHECK-LABEL: _turtle:
+  ; ODR-LABEL: _turtle:
+  ; CHECK-NOT: OUTLINED
+  %1 = alloca i32, align 4
+  %2 = alloca i32, align 4
+  %3 = alloca i32, align 4
+  %4 = alloca i32, align 4
+  store i32 0, i32* %1, align 4
+  store i32 1, i32* %2, align 4
+  store i32 2, i32* %3, align 4
+  store i32 3, i32* %4, align 4
+  ret void
+}
+
 define void @cat() #0 {
   ; CHECK-LABEL: _cat:
   ; CHECK: [[OUTLINED:OUTLINED_FUNCTION_[0-9]+]]
   ; ODR: [[OUTLINED]]
   ; CHECK-NOT: ret
   ; ODR-NOT: ret
   %1 = alloca i32, align 4
   %2 = alloca i32, align 4