Notes on re-materialization.

llvm-svn: 35420

1 files changed, 40 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/README.txt b/llvm/lib/CodeGen/README.txt
index 3f295126fb3..cd40895898e 100644
--- a/llvm/lib/CodeGen/README.txt
+++ b/llvm/lib/CodeGen/README.txt
@@ -1,3 +1,5 @@
+//===---------------------------------------------------------------------===//
+
 Common register allocation / spilling problem:
 
 	mul lr, r4, lr
@@ -25,3 +27,41 @@ and then "merge" mul and mov:
 	mla r4, r3, lr, r4
 
 It also increase the likelyhood the store may become dead.
+
+//===---------------------------------------------------------------------===//
+
+I think we should have a "hasSideEffects" flag (which is automatically set for
+stuff that "isLoad" "isCall" etc), and the remat pass should eventually be able
+to remat any instruction that has no side effects, if it can handle it and if
+profitable.
+
+For now, I'd suggest having the remat stuff work like this:
+
+1. I need to spill/reload this thing.
+2. Check to see if it has side effects.
+3. Check to see if it is simple enough: e.g. it only has one register
+destination and no register input.
+4. If so, clone the instruction, do the xform, etc.
+
+Advantages of this are:
+
+1. the .td file describes the behavior of the instructions, not the way the
+   algorithm should work.
+2. as remat gets smarter in the future, we shouldn't have to be changing the .td
+   files.
+3. it is easier to explain what the flag means in the .td file, because you
+   don't have to pull in the explanation of how the current remat algo works.
+
+Some potential added complexities:
+
+1. Some instructions have to be glued to it's predecessor or successor. All of
+   the PC relative instructions and condition code setting instruction. We could
+   mark them as hasSideEffects, but that's not quite right. PC relative loads
+   from constantpools can be remat'ed, for example. But it requires more than
+   just cloning the instruction. Some instructions can be remat'ed but it
+   expands to more than one instruction. But allocator will have to make a
+   decision.
+
+4. As stated in 3, not as simple as cloning in some cases. The target will have
+   to decide how to remat it. For example, an ARM 2-piece constant generation
+   instruction is remat'ed as a load from constantpool.