Revert "[MC] Always emit relocations for same-section function references"

This reverts commit 5a5ac65.

(cherry picked from commit ae2638d)
(cherry picked from commit 05848b6)

Author: vchuravy

llvm/lib/MC/WinCOFFObjectWriter.cpp

@@ -1190,14 +1190,12 @@ void WinCOFFObjectWriter::reset() {
 bool WinCOFFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
     const MCAssembler &Asm, const MCSymbol &SymA, const MCFragment &FB,
     bool InSet, bool IsPCRel) const {
-  // Don't drop relocations between functions, even if they are in the same text
-  // section. Multiple Visual C++ linker features depend on having the
-  // relocations present. The /INCREMENTAL flag will cause these relocations to
-  // point to thunks, and the /GUARD:CF flag assumes that it can use relocations
-  // to approximate the set of all address taken functions. LLD's implementation
-  // of /GUARD:CF also relies on the existance of these relocations.
+  // MS LINK expects to be able to replace all references to a function with a
+  // thunk to implement their /INCREMENTAL feature.  Make sure we don't optimize
+  // away any relocations to functions.
   uint16_t Type = cast<MCSymbolCOFF>(SymA).getType();
-  if ((Type >> COFF::SCT_COMPLEX_TYPE_SHIFT) == COFF::IMAGE_SYM_DTYPE_FUNCTION)
+  if (Asm.isIncrementalLinkerCompatible() &&
+      (Type >> COFF::SCT_COMPLEX_TYPE_SHIFT) == COFF::IMAGE_SYM_DTYPE_FUNCTION)
     return false;
   return MCObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(Asm, SymA, FB,
                                                                 InSet, IsPCRel);

llvm/test/MC/COFF/diff.s

@@ -1,14 +1,19 @@
 // RUN: llvm-mc -filetype=obj -triple i686-pc-mingw32 %s | llvm-readobj -S --sr --sd - | FileCheck %s
 
-// COFF resolves differences between labels in the same section, unless that
-// label is declared with function type.
-
 .section baz, "xr"
+	.def	X
+	.scl	2;
+	.type	32;
+	.endef
 	.globl	X
 X:
 	mov	Y-X+42,	%eax
 	retl
 
+	.def	Y
+	.scl	2;
+	.type	32;
+	.endef
 	.globl	Y
 Y:
 	retl
@@ -25,11 +30,6 @@ _foobar:                                # @foobar
 # %bb.0:
 	ret
 
-	.globl	_baz
-_baz:
-	calll	_foobar
-	retl
-
 	.data
 	.globl	_rust_crate             # @rust_crate
 	.align	4
@@ -39,15 +39,6 @@ _rust_crate:
 	.long	_foobar-_rust_crate
 	.long	_foobar-_rust_crate
 
-// Even though _baz and _foobar are in the same .text section, we keep the
-// relocation for compatibility with the VC linker's /guard:cf and /incremental
-// flags, even on mingw.
-
-// CHECK:        Name: .text
-// CHECK:        Relocations [
-// CHECK-NEXT:     0x12 IMAGE_REL_I386_REL32 _foobar
-// CHECK-NEXT:   ]
-
 // CHECK:        Name: .data
 // CHECK:        Relocations [
 // CHECK-NEXT:     0x4 IMAGE_REL_I386_DIR32 _foobar