[AsmParser][GAS-compatibility] Ignore an empty 'p2align' directive

[coby]

Summary

GAS ignores the aforementioned issue
this patch aligns LLVM + throws in an appropriate warning

[rnk]

lgtm

lib/MC/MCParser/AsmParser.cpp

3223 ↗

(On Diff #108831)

I'd reword this to be a normal sentence, something like:

// Ignore empty '.p2align' directives for gnu-as compatibility.

llvm/trunk/lib/MC/MCParser/AsmParser.cpp

bool AsmParser::parseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
   SMLoc AlignmentLoc = getLexer().getLoc();
   int64_t Alignment;
   SMLoc MaxBytesLoc;
   bool HasFillExpr = false;
   int64_t FillExpr = 0;
   int64_t MaxBytesToFill = 0;
 
   auto parseAlign = [&]() -> bool {
-    if (checkForValidSection() || parseAbsoluteExpression(Alignment))
+    if (parseAbsoluteExpression(Alignment))
       return true;
     if (parseOptionalToken(AsmToken::Comma)) {
       // The fill expression can be omitted while specifying a maximum number of
       // alignment bytes, e.g:
       //  .align 3,,4
       if (getTok().isNot(AsmToken::Comma)) {
         HasFillExpr = true;
         if (parseAbsoluteExpression(FillExpr))
           return true;
       }
       if (parseOptionalToken(AsmToken::Comma))
         if (parseTokenLoc(MaxBytesLoc) ||
             parseAbsoluteExpression(MaxBytesToFill))
           return true;
     }
     return parseToken(AsmToken::EndOfStatement);
   };
 
+  if (checkForValidSection())
+    return addErrorSuffix(" in directive");
+  // Ignore empty '.p2align' directives for GNU-as compatibility
+  if (IsPow2 && (ValueSize == 1) && getTok().is(AsmToken::EndOfStatement)) {
+    Warning(AlignmentLoc, "p2align directive with no operand(s) is ignored");
+    return parseToken(AsmToken::EndOfStatement);
+  }
   if (parseAlign())
     return addErrorSuffix(" in directive");
 
   // Always emit an alignment here even if we thrown an error.
   bool ReturnVal = false;
 
   // Compute alignment in bytes.
   if (IsPow2) {

llvm/trunk/test/MC/AsmParser/directive_align.s

-# RUN: not llvm-mc -triple i386-apple-darwin9 %s | FileCheck %s
+# RUN: not llvm-mc -triple i386-apple-darwin9 %s 2> %t.err | FileCheck %s
+# RUN: FileCheck < %t.err %s --check-prefix=CHECK-WARN
 
 # CHECK: TEST0:
 # CHECK: .p2align 1
 TEST0:  
         .align 1
 
 # CHECK: TEST1:
 # CHECK: .p2alignl 3, 0x0, 2
 TEST1:  
         .align32 3,,2
 
 # CHECK: TEST2:
 # CHECK: .balign 3, 10
 TEST2:  
         .balign 3,10
+
+# CHECK-WARN: p2align directive with no operand(s) is ignored
+TEST3:  
+        .p2align