[LLVMdev] Modeling volatile register read as intrinsic

[Justin Holewinski]

= Background =

We are trying to introduce a target-dependent intrinsic that compiles to a
read of a volatile register.  In this case, the register holds a perf
counter value, but in general the register can hold any volatile value.
 The problem we have is that it seems this cannot be modeled in LLVM
currently.


= Issue =

The issue we are having is deciding how to represent the intrinsic in LLVM
IR; specifically, the intrinsic properties.  We cannot use IntrNoMem since
that does not model a volatile read of a value.  For example, if we have
two calls to the intrinsic in a function, the optimizers will eliminate one
of the reads and replace it with the value from the other read.

%i1 = tail call i32 @llvm.foo()
%i2 = tail call i32 @llvm.foo()
%i3 = add i32 %i1, %i2

becomes

%i1 = tail call i32 @llvm.foo()
%i2 = add i32 %i1, %i2

Since the read should be volatile, this is not what we want.  On the other
hand, we do not want to use an empty set of properties since this is too
conservative and prevents valid optimizations.  For example, if we do not
use any Intr* properties, the optimizers cannot prove that the intrinsic
does not modify memory.

store i32 42, i32* %ptr
%i1 = tail call i32 @llvm.foo()
%i2 = load i32* %ptr
...
ret i32 %i2

should be optimized to

store i32 42, i32* %ptr
%i1 = tail call i32 @llvm.foo()
...
ret i32 42

but this does not happen since the optimizers cannot prove that the
intrinsic did not modify the value in memory at %ptr.

The IntrReadMem property gets us a bit closer, but still does not model the
volatile property correctly.  The optimizers correctly see the operation as
read-only and can optimize other memory accesses between intrinsic calls
accordingly, but they also assume that the intrinsic reads the same memory
and its return value will not change if a store is not seen between calls
to the intrinsic.  This is the definition of 'readonly' in LLVM IR, which
is not quite what we want.  We still have the same issue as using IntrNoMem
in this regard:

%i1 = tail call i32 @llvm.foo()
%i2 = tail call i32 @llvm.foo()
%i3 = add i32 %i1, %i2

becomes

%i1 = tail call i32 @llvm.foo()
%i2 = add i32 %i1, %i2

unless there is a store instruction between the calls.  For correctness, we
are better off using no properties (over-conservative).


Am I missing something here that could represent this kind of intrinsic?
 It seems like we need a new function attribute readonly_volatile.

-- 

Thanks,

Justin Holewinski
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