Add flag to ArchiveWriter to test GNU64 format more efficiently

Even with the sparse file optimizations the SYM64 test can still be painfully
slow. This unnecessarily slows down devs. It's critical that we test that the
switch to the SYM64 format occurs at 4GB but there isn't any better of a way to
fake the size of the file than sparse files. This change introduces a flag that
allows the cutoff to be arbitrarily set to whatever power of two is desired.
The flag is hidden as it really isn't meant to be used outside this one test.
This is unfortunate but appears necessary, at least until the average hard
drive is much faster.

The changes to the test require some explanation. Prior to this change we knew
that the SYM64 format was being used because the file was simply too large to
have validly handled this case if the SYM64 format were not used. To ensure
that the SYM64 format is still being used I am grepping the file for "SYM64".
Without changing the filename however this would be pointless because "SYM64"
would occur in the file either way. So the filename of the test is also changed
in order to avoid this issue.

Differential Revision: https://reviews.llvm.org/D40632

llvm-svn: 319507

llvm/lib/Object/ArchiveWriter.cpp

@@ -35,6 +35,15 @@
 
 using namespace llvm;
 
+// The SYM64 format is used when an archive's member offsets are larger than
+// 32-bits can hold. The need for this shift in format is detected by
+// writeArchive. To test this we need to generate a file with a member that has
+// an offset larger than 32-bits but this demands a very slow test. To speed
+// the test up we use this flag to pretend like the cutoff happens before
+// 32-bits and instead happens at some much smaller value.
+static cl::opt<int> Sym64Threshold("sym64-threshold", cl::Hidden,
+                                   cl::init(32));
+
 NewArchiveMember::NewArchiveMember(MemoryBufferRef BufRef)
     : Buf(MemoryBuffer::getMemBuffer(BufRef, false)),
       MemberName(BufRef.getBufferIdentifier()) {}
@@ -484,7 +493,7 @@ Error llvm::writeArchive(StringRef ArcName,
     // If LastOffset isn't going to fit in a 32-bit varible we need to switch
     // to 64-bit. Note that the file can be larger than 4GB as long as the last
     // member starts before the 4GB offset.
-    if (LastOffset >> 32 != 0)
+    if (LastOffset >= (1ULL << Sym64Threshold))
       Kind = object::Archive::K_GNU64;
   }
 

llvm/test/Object/archive-GNU64-write.test

@@ -2,11 +2,12 @@
 # REQUIRES: system-linux
 
 # RUN: yaml2obj %s > %t
-# RUN: dd if=%t of=%t bs=1 count=0 seek=2200M
+# RUN: dd if=%t of=%t bs=1 count=0 seek=1M
 # RUN: rm -f %t.lib
 # RUN: cp %t %t2
-# RUN: llvm-ar cr %t.lib %t %t2 %p/Inputs/trivial-object-test.elf-x86-64
+# RUN: llvm-ar -sym64-threshold=19 cr %t.lib %t %t2 %p/Inputs/trivial-object-test.elf-x86-64
 # RUN: llvm-nm --print-armap %t.lib | FileCheck %s
+# RUN: grep SYM64 %t.lib
 
 # Delete temp files. They are too large.
 # RUN: rm -f %t %t2 %t.lib
@@ -28,9 +29,9 @@ Sections:
 # CHECK:      Archive map
 # CHECK-NEXT: main in trivial-object-test.elf-x86-64
 
-# CHECK:    archive-SYM64-write.test.tmp:
+# CHECK:    archive-GNU64-write.test.tmp:
 
-# CHECK:    archive-SYM64-write.test.tmp2:
+# CHECK:    archive-GNU64-write.test.tmp2:
 
 # CHECK:    trivial-object-test.elf-x86-64:
 # CHECK-NEXT:                     U SomeOtherFunction