Remove the `x86_mmx` IR type. (#98505)

It is now translated to `<1 x i64>`, which allows the removal of a bunch
of special casing.

This _incompatibly_ changes the ABI of any LLVM IR function with
`x86_mmx` arguments or returns: instead of passing in mmx registers,
they will now be passed via integer registers. However, the real-world
incompatibility caused by this is expected to be minimal, because Clang
never uses the x86_mmx type -- it lowers `__m64` to either `<1 x i64>`
or `double`, depending on ABI.

This change does _not_ eliminate the SelectionDAG `MVT::x86mmx` type.
That type simply no longer corresponds to an IR type, and is used only
by MMX intrinsics and inline-asm operands.

Because SelectionDAGBuilder only knows how to generate the
operands/results of intrinsics based on the IR type, it thus now
generates the intrinsics with the type MVT::v1i64, instead of
MVT::x86mmx. We need to fix this before the DAG LegalizeTypes, and thus
have the X86 backend fix them up in DAGCombine. (This may be a
short-lived hack, if all the MMX intrinsics can be removed in upcoming
changes.)

Works towards issue #98272.

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 84d624f..fd4ae109 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -2496,7 +2496,9 @@ Error BitcodeReader::parseTypeTableBody() {
       ResultTy = Type::getMetadataTy(Context);
       break;
     case bitc::TYPE_CODE_X86_MMX:   // X86_MMX
-      ResultTy = Type::getX86_MMXTy(Context);
+      // Deprecated: decodes as <1 x i64>
+      ResultTy =
+          llvm::FixedVectorType::get(llvm::IntegerType::get(Context, 64), 1);
       break;
     case bitc::TYPE_CODE_X86_AMX:   // X86_AMX
       ResultTy = Type::getX86_AMXTy(Context);