From e642658480ed0c9cde023a165edca237ba90bd31 Mon Sep 17 00:00:00 2001 From: Nick Lewycky Date: Mon, 15 Dec 2008 01:34:58 +0000 Subject: 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 --- lib/Bitcode/Reader/BitcodeReader.cpp | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'lib/Bitcode/Reader/BitcodeReader.cpp') diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp index f06c61de41..2d994d4b13 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) -- cgit v1.2.3-18-g5258