[LLVMdev] all LLVM Instructions that may write to memory -- other than StoreInst?

John Criswell
Fri, 21 Jan 2011 22:33:59 -0000

On 1/21/11 2:50 PM, Chuck Zhao wrote:
> I need to figure out all LLVM Instructions that may write to memory.
>
> In http://llvm.org/docs/tutorial/OCamlLangImpl7.html, it mentions that
> "In LLVM, all memory accesses are explicit with load/store
> instructions, and it is carefully designed not to have (or need) an
> "address-of" operator."
>
> I take this as "StoreInst is the only one that writes to memory".

There are intrinsic functions which write to memory also, such as
memcpy.
>
> However, this doesn't seem to be enough.

Your observation is correct.  Strictly speaking, any instruction can
write to memory after code generation because it may access a stack
spill slot or a function parameter which the ABI places on the stack.

When the Language Reference Manual talks about writing to memory, it is
talking about writing to memory that is visible at the LLVM IR level.
The stack frame is invisible at the LLVM IR level.  Put another way,
"memory" is a set of memory locations which can be explicitly accessed
by LLVM load and store instructions and are not in SSA form; it is not
all of the memory within the computer.

If you're interested in finding instructions that write to RAM
(including writes to stack spill slots), it may be better to work on
Machine Instructions within the code generator framework.

-- John T.


>
> Consider:
> ...
> int a, b, d;
> d = a + b;
> ...
>
> The above code is turned into LLVM IR:
>    %0 = load i32* @a, align 4
>    %1 = load i32* @b, align 4
>    %2 = add nsw i32 %1, %0
>    store i32 %2, i32* @d, align 4
>
> Is it possible that temps such as %0, %1 and/or %2 will NOT being register
allocated later in the compilation stage, and thus left in memory?
>
> The above code, when converted back to C level, looks like this:
> ...
>    unsigned int llvm_cbe_tmp__6;
>    unsigned int llvm_cbe_tmp__7;
>    unsigned int llvm_cbe_tmp__8;
>    unsigned int llvm_cbe_tmp__9;
>
>    llvm_cbe_tmp__6 = *(&a);
>    llvm_cbe_tmp__7 = *(&b);
>    llvm_cbe_tmp__8 = ((unsigned int )(((unsigned int )llvm_cbe_tmp__7) +
((unsigned int )llvm_cbe_tmp__6)));
>    *(&d) = llvm_cbe_tmp__8;
>    llvm_cbe_tmp__9 =  /*tail*/ printf(((&_OC_str.array[((signed int
)0u)])), llvm_cbe_tmp__8);
> ...
>
> It seems the compiler-generated temps are _actually_ left on stack, and
writes to them are actually writes to stack memory (via load, add, ...).
>
>
>
> I am confused here.
> Could somebody help to clarify it?
>
> Thank you
>
> Chuck
>
>
>

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