[IR] Fix mayReadFromMemory() for writeonly calls

[yrouban]

Summary

Current implementation of Instruction::mayReadFromMemory() returns !doesNotAccessMemory() which is !ReadNone. This does not take into account that the writeonly attribute also indicates that the call does not read from memory.
The patch proposes to change the predicate to !doesNotReadMemory() that reflects the intended behavior.

[lebedev.ri]

Can this has test coverage?

[yrouban]

Added a simple early-cse test.

[yrouban]

Fixed llvm/test/CodeGen/AMDGPU/llvm.amdgcn.ds.gws.init.ll as Matt suggested

[arsenm]

LGTM with minor test change

llvm/test/Transforms/EarlyCSE/writeonly.ll

8

I would loosen this to just CHECK-NOT: store in case of future IR syntax changes

llvm/lib/IR/Instruction.cpp

bool Instruction::mayReadFromMemory() const {
   case Instruction::AtomicCmpXchg:
   case Instruction::AtomicRMW:
   case Instruction::CatchPad:
   case Instruction::CatchRet:
     return true;
   case Instruction::Call:
   case Instruction::Invoke:
   case Instruction::CallBr:
-    return !cast<CallBase>(this)->doesNotAccessMemory();
+    return !cast<CallBase>(this)->doesNotReadMemory();
   case Instruction::Store:
     return !cast<StoreInst>(this)->isUnordered();
   }
 }
 
 bool Instruction::mayWriteToMemory() const {
   switch (getOpcode()) {
   default: return false;

llvm/test/CodeGen/AMDGPU/llvm.amdgcn.ds.gws.init.ll

 ; LOOP: s_mov_b32 m0, 0
 ; LOOP: s_setreg_imm32_b32
 ; LOOP: ds_gws_init v{{[0-9]+}} offset:10 gds
 ; LOOP: s_cbranch_scc1
 
 ; LOOP: s_mov_b32 m0, -1
 ; LOOP: ds_write_b32
 define amdgpu_kernel void @gws_init_save_m0_init_constant_offset(i32 %val) #0 {
-  store i32 1, i32 addrspace(3)* @lds
+  store volatile i32 1, i32 addrspace(3)* @lds
   call void @llvm.amdgcn.ds.gws.init(i32 %val, i32 10)
   store i32 2, i32 addrspace(3)* @lds
   ret void
 }
 
 ; GCN-LABEL: {{^}}gws_init_lgkmcnt:
 ; NOLOOP: s_mov_b32 m0, 0{{$}}
 ; NOLOOP: ds_gws_init v0 gds{{$}}

llvm/test/Transforms/EarlyCSE/writeonly.ll

+; RUN: opt -S -early-cse < %s | FileCheck %s
+
+@var = global i32 undef
+declare void @foo() nounwind
+
+define void @test() {
+; CHECK-LABEL: @test(
+; CHECK-NOT: store i32 1, i32* @var
+  store i32 1, i32* @var
+; CHECK: call void @foo()
+  call void @foo() writeonly
+; CHECK: store i32 2, i32* @var
+  store i32 2, i32* @var
+  ret void
+}