Warn on align directive with non-zero fill value in virtual sections

This patch warns when an align directive with a non-zero fill value is
used in a virtual section. The fill value is also set to zero,
preventing an assertion in MCAssembler::writeSectionData for the case
of MCFragment::FT_Align from tripping.

Author: luismarques

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -3392,6 +3392,7 @@ bool AsmParser::parseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
   bool HasFillExpr = false;
   int64_t FillExpr = 0;
   int64_t MaxBytesToFill = 0;
+  SMLoc FillExprLoc;
 
   auto parseAlign = [&]() -> bool {
     if (parseAbsoluteExpression(Alignment))
@@ -3402,7 +3403,7 @@ bool AsmParser::parseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
       //  .align 3,,4
       if (getTok().isNot(AsmToken::Comma)) {
         HasFillExpr = true;
-        if (parseAbsoluteExpression(FillExpr))
+        if (parseTokenLoc(FillExprLoc) || parseAbsoluteExpression(FillExpr))
           return true;
       }
       if (parseOptionalToken(AsmToken::Comma))
@@ -3451,6 +3452,17 @@ bool AsmParser::parseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
     }
   }
 
+  if (HasFillExpr) {
+    MCSection *Sec = getStreamer().getCurrentSectionOnly();
+    if (Sec && Sec->isVirtualSection()) {
+      ReturnVal |=
+          Warning(FillExprLoc, "ignoring non-zero fill value in " +
+                                   Sec->getVirtualSectionKind() + " section '" +
+                                   Sec->getName() + "'");
+      FillExpr = 0;
+    }
+  }
+
   // Diagnose non-sensical max bytes to align.
   if (MaxBytesLoc.isValid()) {
     if (MaxBytesToFill < 1) {

llvm/test/MC/ELF/nobits-non-zero-value.s

@@ -12,5 +12,8 @@
 # CHECK: {{.*}}.s:[[#@LINE+1]]:3: error: SHT_NOBITS section '.bss' cannot have instructions
   addb %al,(%rax)
 
+# CHECK: {{.*}}.s:[[#@LINE+1]]:11: warning: ignoring non-zero fill value in SHT_NOBITS section '.bss'
+.align 4, 42
+
 # CHECK: <unknown>:0: error: SHT_NOBITS section '.bss' cannot have non-zero initializers
   .long 1