[llvm-dev] fast-math-flags propagated to DAG nodes from IR instructions

[Sanjay Patel via llvm-dev]

Hi backend maintainers/hackers,

With r247815:
http://llvm.org/viewvc/llvm-project?view=revision&revision=247815

...we're propagating fast-math-flags (FMF) from IR to DAG nodes by default.
This has been reverted before...
But it's been one week since the commit and I haven't heard any complaints
yet, so I'm hoping it sticks this time.

If you have target-specific DAG combines on FP nodes, you probably want to
propagate the optimization flags from incoming nodes to nodes that you are
creating. Otherwise, you may fail to get CSE optimizations that happened
before this change. See the test case in the patch for an example of how
that might manifest.

The code change when creating a node that should have FMF looks something
like this:

-      Tmp1 = DAG.getNode(ISD::FADD, dl, VT, Node->getOperand(0), Tmp1);
+      Tmp1 = DAG.getNode(ISD::FADD, dl, VT, Node->getOperand(0),
Tmp1, Node->getFlags());


FMF only applies to binary FP ops at the moment, but as noted in the commit
message, the plan is to extend the flags to more DAG node types.

Going forward, we can use the node-level flags to trigger optimizations
that the global target settings (eg, DAG.getTarget().Options.UnsafeFPMath)
might not allow. And some day, we may be able to remove global FP
optimization flags entirely.
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