[IR][ModRef] Introduce `errno` memory location

Model C/C++ `errno` macro by adding a corresponding `errno`
memory location kind to the IR. Preliminary work to separate
`errno` writes from other memory accesses, to the benefit of
alias analyses and optimization correctness.

Previous discussion: https://discourse.llvm.org/t/rfc-modelling-errno-memory-effects/82972.

1 files changed, 23 insertions, 5 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 1a09e80..d687495 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -1937,8 +1937,7 @@ static void addRawAttributeValue(AttrBuilder &B, uint64_t Val) {
 }
 
 /// This fills an AttrBuilder object with the LLVM attributes that have
-/// been decoded from the given integer. This function must stay in sync with
-/// 'encodeLLVMAttributesForBitcode'.
+/// been decoded from the given integer.
 static void decodeLLVMAttributesForBitcode(AttrBuilder &B,
                                            uint64_t EncodedAttrs,
                                            uint64_t AttrIdx) {
@@ -2398,9 +2397,28 @@ Error BitcodeReader::parseAttributeGroupBlock() {
             B.addUWTableAttr(UWTableKind(Record[++i]));
           else if (Kind == Attribute::AllocKind)
             B.addAllocKindAttr(static_cast<AllocFnKind>(Record[++i]));
-          else if (Kind == Attribute::Memory)
-            B.addMemoryAttr(MemoryEffects::createFromIntValue(Record[++i]));
-          else if (Kind == Attribute::Captures)
+          else if (Kind == Attribute::Memory) {
+            uint64_t EncodedME = Record[++i];
+            const uint8_t Version = (EncodedME >> 56);
+            if (Version == 0) {
+              // Errno memory location was previously encompassed into default
+              // memory. Ensure this is taken into account while reconstructing
+              // the memory attribute prior to its introduction.
+              ModRefInfo ArgMem = ModRefInfo((EncodedME >> 0) & 3);
+              ModRefInfo InaccessibleMem = ModRefInfo((EncodedME >> 2) & 3);
+              ModRefInfo OtherMem = ModRefInfo((EncodedME >> 4) & 3);
+              auto ME = MemoryEffects::inaccessibleMemOnly(InaccessibleMem) |
+                        MemoryEffects::argMemOnly(ArgMem) |
+                        MemoryEffects::errnoMemOnly(OtherMem) |
+                        MemoryEffects::otherMemOnly(OtherMem);
+              B.addMemoryAttr(ME);
+            } else {
+              // Construct the memory attribute directly from the encoded base
+              // on newer versions.
+              B.addMemoryAttr(MemoryEffects::createFromIntValue(
+                  EncodedME & 0x00FFFFFFFFFFFFFFULL));
+            }
+          } else if (Kind == Attribute::Captures)
             B.addCapturesAttr(CaptureInfo::createFromIntValue(Record[++i]));
           else if (Kind == Attribute::NoFPClass)
             B.addNoFPClassAttr(