Introducing nocapture, a parameter attribute for pointers to indicate that the

callee will not introduce any new aliases of that pointer.

The attributes had all bits allocated already, so I decided to collapse
alignment. Alignment was previously stored as a 16-bit integer from bits 16 to
32 of the attribute, but it was required to be a power of 2. Now it's stored in
log2 encoded form in five bits from 16 to 21. That gives us 11 more bits of
space.

You may have already noticed that you only need four bits to encode a 16-bit
power of two, so why five bits? Because the AsmParser accepted 32-bit
alignments, even though we couldn't store them (they were silently discarded).
Now we can store them in memory, but not in the bitcode.

The bitcode format was already storing these as 64-bit VBR integers. So, the
bitcode format stays the same, keeping the alignment values stored as 16 bit
raw values. There's some hideous code in the reader and writer that deals with
this, waiting to be ripped out the moment we run out of bits again and have to
replace the parameter attributes table encoding.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61019 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 24 insertions, 1 deletions

diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index f06c61d..2d994d4 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -362,6 +362,20 @@ bool BitcodeReader::ParseAttributeBlock() {
       Attributes RetAttribute = Attribute::None;
       Attributes FnAttribute = Attribute::None;
       for (unsigned i = 0, e = Record.size(); i != e; i += 2) {
+        // FIXME: remove in LLVM 3.0
+        // The alignment is stored as a 16-bit raw value from bits 31--16.
+        // We shift the bits above 31 down by 11 bits.
+
+        unsigned Alignment = (Record[i+1] & (0xffffull << 16)) >> 16;
+        if (Alignment && !isPowerOf2_32(Alignment))
+          return Error("Alignment is not a power of two.");
+
+        Attributes ReconstitutedAttr = Record[i+1] & 0xffff;
+        if (Alignment)
+          ReconstitutedAttr |= Attribute::constructAlignmentFromInt(Alignment);
+        ReconstitutedAttr |= (Record[i+1] & (0xffffull << 32)) >> 11;
+        Record[i+1] = ReconstitutedAttr;
+
         if (Record[i] == 0)
           RetAttribute = Record[i+1];
         else if (Record[i] == ~0U)
diff --git a/lib/Bitcode/Writer/BitcodeWriter.cpp b/lib/Bitcode/Writer/BitcodeWriter.cpp
index 279e447..0555ed9 100644
--- a/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -122,7 +122,16 @@ static void WriteAttributeTable(const ValueEnumerator &VE,
     for (unsigned i = 0, e = A.getNumSlots(); i != e; ++i) {
       const AttributeWithIndex &PAWI = A.getSlot(i);
       Record.push_back(PAWI.Index);
-      Record.push_back(PAWI.Attrs);
+
+      // FIXME: remove in LLVM 3.0
+      // Store the alignment in the bitcode as a 16-bit raw value instead of a
+      // 5-bit log2 encoded value. Shift the bits above the alignment up by
+      // 11 bits.
+      uint64_t FauxAttr = PAWI.Attrs & 0xffff;
+      FauxAttr |= (1ull<<16)<<((PAWI.Attrs & Attribute::Alignment) >> 16);
+      FauxAttr |= (PAWI.Attrs & (0x3FFull << 21)) << 11;
+
+      Record.push_back(FauxAttr);
     }
     
     Stream.EmitRecord(bitc::PARAMATTR_CODE_ENTRY, Record);