Never use .lcomm on platforms where it does not accept an alignment

argument.  Instead, use a pair of .local and .comm directives.

This avoids spurious differences between binaries built by the
integrated assembler vs. those built by the external assembler,
since the external assembler may impose alignment requirements
on .lcomm symbols where the integrated assembler does not.

llvm-svn: 168704

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -308,8 +308,13 @@ void AsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
       return;
     }
 
-    if (Align == 1 ||
-        MAI->getLCOMMDirectiveAlignmentType() != LCOMM::NoAlignment) {
+    // Use .lcomm only if it supports user-specified alignment.
+    // Otherwise, while it would still be correct to use .lcomm in some
+    // cases (e.g. when Align == 1), the external assembler might enfore
+    // some -unknown- default alignment behavior, which could cause
+    // spurious differences between external and integrated assembler.
+    // Prefer to simply fall back to .local / .comm in this case.
+    if (MAI->getLCOMMDirectiveAlignmentType() != LCOMM::NoAlignment) {
       // .lcomm _foo, 42
       OutStreamer.EmitLocalCommonSymbol(GVSym, Size, Align);
       return;

llvm/test/CodeGen/ARM/2010-12-15-elf-lcomm.ll

@@ -10,7 +10,8 @@
 @STRIDE = internal global i32 8
 
 ; ASM:          .type   array00,%object         @ @array00
-; ASM-NEXT:     .lcomm  array00,80
+; ASM-NEXT:     .local  array00
+; ASM-NEXT:     .comm   array00,80,1
 ; ASM-NEXT:     .type   _MergedGlobals,%object  @ @_MergedGlobals
 
 

llvm/test/CodeGen/ARM/elf-lcomm-align.ll

@@ -4,8 +4,9 @@
 @c = internal global i8 0, align 1
 @x = internal global i32 0, align 4
 
-; CHECK: .lcomm c,1
-; .lcomm doesn't support alignment.
+; .lcomm doesn't support alignment, so we always use .local/.comm.
+; CHECK: .local c
+; CHECK-NEXT: .comm c,1,1
 ; CHECK: .local x
 ; CHECK-NEXT: .comm x,4,4