[IR] Convert from nocapture to captures(none) (#123181)

This PR removes the old `nocapture` attribute, replacing it with the new
`captures` attribute introduced in #116990. This change is
intended to be essentially NFC, replacing existing uses of `nocapture`
with `captures(none)` without adding any new analysis capabilities.
Making use of non-`none` values is left for a followup.

Some notes:
* `nocapture` will be upgraded to `captures(none)` by the bitcode
   reader.
* `nocapture` will also be upgraded by the textual IR reader. This is to
   make it easier to use old IR files and somewhat reduce the test churn in
   this PR.
* Helper APIs like `doesNotCapture()` will check for `captures(none)`.
* MLIR import will convert `captures(none)` into an `llvm.nocapture`
   attribute. The representation in the LLVM IR dialect should be updated
   separately.

1 files changed, 12 insertions, 3 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 551dfd4..e16e8a0 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -1865,7 +1865,7 @@ static uint64_t getRawAttributeMask(Attribute::AttrKind Val) {
   case Attribute::StackProtect:    return 1 << 14;
   case Attribute::StackProtectReq: return 1 << 15;
   case Attribute::Alignment:       return 31 << 16;
-  case Attribute::NoCapture:       return 1 << 21;
+  // 1ULL << 21 is NoCapture, which is upgraded separately.
   case Attribute::NoRedZone:       return 1 << 22;
   case Attribute::NoImplicitFloat: return 1 << 23;
   case Attribute::Naked:           return 1 << 24;
@@ -1986,6 +1986,12 @@ static void decodeLLVMAttributesForBitcode(AttrBuilder &B,
       B.addMemoryAttr(ME);
   }
 
+  // Upgrade nocapture to captures(none).
+  if (Attrs & (1ULL << 21)) {
+    Attrs &= ~(1ULL << 21);
+    B.addCapturesAttr(CaptureInfo::none());
+  }
+
   addRawAttributeValue(B, Attrs);
 }
 
@@ -2098,8 +2104,6 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::NoBuiltin;
   case bitc::ATTR_KIND_NO_CALLBACK:
     return Attribute::NoCallback;
-  case bitc::ATTR_KIND_NO_CAPTURE:
-    return Attribute::NoCapture;
   case bitc::ATTR_KIND_NO_DIVERGENCE_SOURCE:
     return Attribute::NoDivergenceSource;
   case bitc::ATTR_KIND_NO_DUPLICATE:
@@ -2349,6 +2353,11 @@ Error BitcodeReader::parseAttributeGroupBlock() {
               upgradeOldMemoryAttribute(ME, EncodedKind))
             continue;
 
+          if (EncodedKind == bitc::ATTR_KIND_NO_CAPTURE) {
+            B.addCapturesAttr(CaptureInfo::none());
+            continue;
+          }
+
           if (Error Err = parseAttrKind(EncodedKind, &Kind))
             return Err;