llvm-reduce: Add values to return reduction (#132686)

In void functions, try to replace instruction uses
with a new non-void return. If the return type matches
the instruction, also try to directly return it.

This handles most of the cases, but doesn't try to handle
all of the weird exception related terminators.

Also doesn't try to replace argument uses, although it could. We
could also handle cases where we can insert a simple cast to an
original return value. I didn't think too hard about where to put this
in the default pass order. In many cases it obviates the need for most
of the CFG folds, but I've left it near the end initially.

I also think this is too aggressive about removing dead code, and
should leave existing dead code alone. I'm also not sure why we have
both "removeUnreachableBlocks" and EliminateUnreachableBlocks" in Utils.

Fixes #66039, fixes #107327

Author: arsenm

llvm/include/llvm/IR/Function.h

@@ -1048,6 +1048,14 @@ class LLVM_ABI Function : public GlobalObject, public ilist_node<Function> {
   void setValueSubclassDataBit(unsigned Bit, bool On);
 };
 
+namespace CallingConv {
+
+// TODO: Need similar function for support of argument in position. General
+// version on FunctionType + Attributes + CallingConv::ID?
+LLVM_READNONE
+bool supportsNonVoidReturnType(CallingConv::ID CC);
+} // namespace CallingConv
+
 /// Check whether null pointer dereferencing is considered undefined behavior
 /// for a given function or an address space.
 /// Null pointer access in non-zero address space is not considered undefined.

llvm/lib/IR/Function.cpp

@@ -1176,3 +1176,85 @@ bool llvm::NullPointerIsDefined(const Function *F, unsigned AS) {
 
   return false;
 }
+
+bool llvm::CallingConv::supportsNonVoidReturnType(CallingConv::ID CC) {
+  switch (CC) {
+  case CallingConv::C:
+  case CallingConv::Fast:
+  case CallingConv::Cold:
+  case CallingConv::GHC:
+  case CallingConv::HiPE:
+  case CallingConv::AnyReg:
+  case CallingConv::PreserveMost:
+  case CallingConv::PreserveAll:
+  case CallingConv::Swift:
+  case CallingConv::CXX_FAST_TLS:
+  case CallingConv::Tail:
+  case CallingConv::CFGuard_Check:
+  case CallingConv::SwiftTail:
+  case CallingConv::PreserveNone:
+  case CallingConv::X86_StdCall:
+  case CallingConv::X86_FastCall:
+  case CallingConv::ARM_APCS:
+  case CallingConv::ARM_AAPCS:
+  case CallingConv::ARM_AAPCS_VFP:
+  case CallingConv::MSP430_INTR:
+  case CallingConv::X86_ThisCall:
+  case CallingConv::PTX_Device:
+  case CallingConv::SPIR_FUNC:
+  case CallingConv::Intel_OCL_BI:
+  case CallingConv::X86_64_SysV:
+  case CallingConv::Win64:
+  case CallingConv::X86_VectorCall:
+  case CallingConv::DUMMY_HHVM:
+  case CallingConv::DUMMY_HHVM_C:
+  case CallingConv::X86_INTR:
+  case CallingConv::AVR_INTR:
+  case CallingConv::AVR_SIGNAL:
+  case CallingConv::AVR_BUILTIN:
+    return true;
+  case CallingConv::AMDGPU_KERNEL:
+  case CallingConv::SPIR_KERNEL:
+  case CallingConv::AMDGPU_CS_Chain:
+  case CallingConv::AMDGPU_CS_ChainPreserve:
+    return false;
+  case CallingConv::AMDGPU_VS:
+  case CallingConv::AMDGPU_HS:
+  case CallingConv::AMDGPU_GS:
+  case CallingConv::AMDGPU_PS:
+  case CallingConv::AMDGPU_CS:
+  case CallingConv::AMDGPU_LS:
+  case CallingConv::AMDGPU_ES:
+  case CallingConv::MSP430_BUILTIN:
+  case CallingConv::AArch64_VectorCall:
+  case CallingConv::AArch64_SVE_VectorCall:
+  case CallingConv::WASM_EmscriptenInvoke:
+  case CallingConv::AMDGPU_Gfx:
+  case CallingConv::M68k_INTR:
+  case CallingConv::AArch64_SME_ABI_Support_Routines_PreserveMost_From_X0:
+  case CallingConv::AArch64_SME_ABI_Support_Routines_PreserveMost_From_X2:
+  case CallingConv::M68k_RTD:
+  case CallingConv::GRAAL:
+  case CallingConv::ARM64EC_Thunk_X64:
+  case CallingConv::ARM64EC_Thunk_Native:
+  case CallingConv::RISCV_VectorCall:
+  case CallingConv::AArch64_SME_ABI_Support_Routines_PreserveMost_From_X1:
+  case CallingConv::RISCV_VLSCall_32:
+  case CallingConv::RISCV_VLSCall_64:
+  case CallingConv::RISCV_VLSCall_128:
+  case CallingConv::RISCV_VLSCall_256:
+  case CallingConv::RISCV_VLSCall_512:
+  case CallingConv::RISCV_VLSCall_1024:
+  case CallingConv::RISCV_VLSCall_2048:
+  case CallingConv::RISCV_VLSCall_4096:
+  case CallingConv::RISCV_VLSCall_8192:
+  case CallingConv::RISCV_VLSCall_16384:
+  case CallingConv::RISCV_VLSCall_32768:
+  case CallingConv::RISCV_VLSCall_65536:
+    return true;
+  default:
+    return false;
+  }
+
+  llvm_unreachable("covered callingconv switch");
+}

llvm/test/tools/llvm-reduce/reduce-instructions-to-return-nonvoid-noncallee-use.ll

@@ -0,0 +1,27 @@
+; Make sure we don't break on non-callee uses of funtions with a
+; non-void return type.
+
+; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=instructions-to-return --test FileCheck --test-arg --check-prefix=INTERESTING --test-arg %s --test-arg --input-file %s -o %t
+; RUN: FileCheck --check-prefix=RESULT %s < %t
+
+; INTERESTING-LABEL: @interesting(
+; INTERESTING: %inttoptr = inttoptr i64
+
+; RESULT-LABEL: define ptr @interesting(i64 %arg) {
+; RESULT-NEXT: %inttoptr = inttoptr i64 %arg to ptr
+; RESULT-NEXT:  ret ptr %inttoptr
+define void @interesting(i64 %arg) {
+  %inttoptr = inttoptr i64 %arg to ptr
+  %load = load i32, ptr %inttoptr
+  ret void
+}
+
+declare i32 @func(ptr)
+
+; RESULT-LABEL: define i32 @caller() {
+; RESULT-NEXT: %call = call i32 @func(ptr @interesting)
+; RESULT-NEXT: ret i32 %call
+define void @caller() {
+  %call = call i32 @func(ptr @interesting)
+  ret void
+}