From 9ba0e2ba87a5af3b863d9a5f6f620c9b7583d0f4 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Wed, 14 Oct 2009 00:02:01 +0000 Subject: This remat entry is basically done. There are hooks to allow targets to remat non-load instructions as loads, and the remat code now uses the UnmodeledSideEffects flags, MachineMemOperands, and similar things to decide which instructions are valid for rematerialization. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84060 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/CodeGen/README.txt | 38 -------------------------------------- 1 file changed, 38 deletions(-) (limited to 'lib') diff --git a/lib/CodeGen/README.txt b/lib/CodeGen/README.txt index 8e9ead7f66..b655dda411 100644 --- a/lib/CodeGen/README.txt +++ b/lib/CodeGen/README.txt @@ -30,44 +30,6 @@ 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. - -//===---------------------------------------------------------------------===// - bb27 ... ... %reg1037 = ADDri %reg1039, 1 -- cgit v1.2.3