[PATCH] D20116: Add nosideeffects function attribute

[Eli Friedman via llvm-commits]

eli.friedman added inline comments.

================
Comment at: docs/LangRef.rst:1405
@@ +1404,3 @@
+    state that isn't accessbile from the IR (e.g. floating-point exception
+    registers).  Functions with the nosideeffects attribute can be safely
+    speculated.
----------------
The floating point status register is a weird example.  The floating point status register is basically memory; it isn't actually addressable on most processors, but it behaves like a hidden global in every other way.

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Comment at: utils/TableGen/CodeGenIntrinsics.h:113
@@ +112,3 @@
+    return hasSideEffectsProperty ||
+      (ModRef >= CodeGenIntrinsic::ReadWriteMem && !hasNoSideEffectsProperty);
+  }
----------------
tstellarAMD wrote:
> This a problem with the current definitions of TableGen's intrinsic properties.  Any intrinsic with IntrNoMem, IntrReadMem, IntrWriteMem, or IntrArgMemOnly is defined as having no side-effects.  The goal with this patch is to make it possible to have an intrinsic, like memcpy which only reads/writes arg memory, but may have other sideeffects. 
Oh, I see, this is an existing problem. :(  I'd definitely like to see this resolved before you start changing optimizations to use this flag, but I guess you can change it in a followup.

Not that it helps memcpy in particular, but it might be worth considering some approach which allows one to say "this intrinsic has no side-effects if the pointer arguments are dereferenceable(n)".


http://reviews.llvm.org/D20116