Emit function alias to data as a function symbol

[eugenis]

Summary

CFI emits jump slots for indirect functions as a byte array constant, and declares function-typed aliases to these constants.
This change fixes AsmPrinter to emit these aliases as function symbols, and not data symbols.

It does it by looking at the pointee type of the alias, which feels like the wrong thing to do. Does anyone have any better ideas?

[eugenis]

s/SFI/CFI/

[pcc]

Eventually there should be a flag or attribute on the GlobalAlias that specifies the type of symbol to create. That can probably happen later though. For now the source of truth is the pointee type, and I am aware that we are using it elsewhere (e.g. in the COFF writer).

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1192

This will always be true (GlobalValue::getType returns a PointerType *).

1194

Is this check needed?

[eugenis]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1192

Good point. Removed.

1194

It's not strictly necessary, but without it, we would insert extra

.type alias,@function

(in .S output) when the aliasee is already a function.

[pcc]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1194

It shouldn't do any harm to emit that directive redundantly, so I'd prefer to remove it, as it makes the code easier to reason about.

[eugenis]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1194

sure, done.

[pcc]

LGTM with nit

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1190

Comment is no longer accurate.

[eugenis]

r254674

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1190

fixed

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

for (const auto &Alias : M.aliases()) {
 
     if (Alias.hasExternalLinkage() || !MAI->getWeakRefDirective())
       OutStreamer->EmitSymbolAttribute(Name, MCSA_Global);
     else if (Alias.hasWeakLinkage() || Alias.hasLinkOnceLinkage())
       OutStreamer->EmitSymbolAttribute(Name, MCSA_WeakReference);
     else
       assert(Alias.hasLocalLinkage() && "Invalid alias linkage");
 
+    // If the alias has a function type and the aliasee is not a function,
+    // explicitly switch the symbol type to function.
+    if (Alias.getType()->isPointerTy() &&
+        Alias.getType()->getPointerElementType()->isFunctionTy() &&
+        !isa<Function>(Alias.getAliasee()))
+      OutStreamer->EmitSymbolAttribute(Name, MCSA_ELF_TypeFunction);
+
     EmitVisibility(Name, Alias.getVisibility());
 
     // Emit the directives as assignments aka .set:
     OutStreamer->EmitAssignment(Name, lowerConstant(Alias.getAliasee()));
 
     // If the aliasee does not correspond to a symbol in the output, i.e. the
     // alias is not of an object or the aliased object is private, then set the
     // size of the alias symbol from the type of the alias. We don't do this in

test/CodeGen/Generic/function-alias.ll

+; RUN: llc < %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; "data" constant
+@0 = private constant <{ i8, i8 }> <{i8 15, i8 11}>, section ".text"
+
+; function-typed alias
+@ud2 = alias void (), bitcast (<{ i8, i8 }>* @0 to void ()*)
+
+; Check that "ud2" is emitted as a function symbol.
+; CHECK: .type{{.*}}ud2,@function