[PATCH] D12682: [MC/ELF] Accept zero for .align directive

[Davide Italiano via llvm-commits]

davide created this revision.
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.align directive refuses alignment 0 -- a comment in the code hints this is done for GNU as compatibility, but it seems GNU as accepts .align 0 (and silently rounds up alignment to 1). This patch is an attempt to fix.

$ ./llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu blah.s -o blah
blah.s:4:8: error: alignment must be a power of 2
.align 0
       ^
$ as blah.s -o blah
$

$ cat blah.s
.global _start
.global bar

.align 0
.text
_start:
bar:
  movl $bar, %edx


Repository:
  rL LLVM

http://reviews.llvm.org/D12682

Files:
  lib/MC/MCParser/AsmParser.cpp
  test/MC/ELF/align-zero.s

Index: test/MC/ELF/align-zero.s
===================================================================
--- test/MC/ELF/align-zero.s
+++ test/MC/ELF/align-zero.s
@@ -0,0 +1,24 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | llvm-readobj -s | FileCheck %s
+
+// Test that an alignment of zero is accepted.
+
+  nop
+  .align 0
+  .text
+
+// CHECK: Section {
+// CHECK:   Index: 2
+// CHECK-NEXT:   Name: .text (1)
+// CHECK-NEXT:   Type: SHT_PROGBITS (0x1)
+// CHECK-NEXT:   Flags [ (0x6)
+// CHECK-NEXT:     SHF_ALLOC (0x2)
+// CHECK-NEXT:     SHF_EXECINSTR (0x4)
+// CHECK-NEXT:   ]
+// CHECK-NEXT:   Address: 0x0
+// CHECK-NEXT:   Offset: 0x40
+// CHECK-NEXT:   Size: 1
+// CHECK-NEXT:   Link: 0
+// CHECK-NEXT:   Info: 0
+// CHECK-NEXT:   AddressAlignment: 4
+// CHECK-NEXT:   EntrySize: 0
+// CHECK-NEXT: }
Index: lib/MC/MCParser/AsmParser.cpp
===================================================================
--- lib/MC/MCParser/AsmParser.cpp
+++ lib/MC/MCParser/AsmParser.cpp
@@ -2706,7 +2706,11 @@
 
     Alignment = 1ULL << Alignment;
   } else {
-    // Reject alignments that aren't a power of two, for gas compatibility.
+    // Reject alignments that aren't either a power of two or zero,
+    // for gas compatibility. Alignment of zero is silently rounded
+    // up to one.
+    if (Alignment == 0)
+      Alignment = 1;
     if (!isPowerOf2_64(Alignment))
       Error(AlignmentLoc, "alignment must be a power of 2");
   }


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