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author | Dan Gohman <gohman@apple.com> | 2009-10-14 00:02:01 +0000 |
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committer | Dan Gohman <gohman@apple.com> | 2009-10-14 00:02:01 +0000 |
commit | df777edea90cce690bf1c0ddf8b0d40446f9202f (patch) | |
tree | dd978a849ca9f12abc36f7d202573731f157a82b /llvm/lib/CodeGen/README.txt | |
parent | 0aa63c97cea4cbaa486c50a65bd2ebdaa9f5bf4e (diff) | |
download | llvm-df777edea90cce690bf1c0ddf8b0d40446f9202f.zip llvm-df777edea90cce690bf1c0ddf8b0d40446f9202f.tar.gz llvm-df777edea90cce690bf1c0ddf8b0d40446f9202f.tar.bz2 |
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.
llvm-svn: 84060
Diffstat (limited to 'llvm/lib/CodeGen/README.txt')
-rw-r--r-- | llvm/lib/CodeGen/README.txt | 38 |
1 files changed, 0 insertions, 38 deletions
diff --git a/llvm/lib/CodeGen/README.txt b/llvm/lib/CodeGen/README.txt index 8e9ead7..b655dda 100644 --- a/llvm/lib/CodeGen/README.txt +++ b/llvm/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 |