[PATCH] D48485: [InstCombine] allow shl+mul combos with shuffle (select) fold (PR37806)

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


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Comment at: lib/Transforms/InstCombine/InstCombineVectorOps.cpp:1177
+    // each vector element to determine if it is safe to keep that flag.
+    if (Opc0 == Instruction::Mul && Opc1 == Instruction::Shl) {
+      C1 = ConstantExpr::getShl(ConstantInt::get(C1->getType(), 1), C1);
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RKSimon wrote:
> Is this going to scale well? There's likely to be a lot of 'similar' cases (ADD x,x -> SHL x,1 etc.)
That's what I wasn't sure about. I was guessing that add/sub was the common case, and we already canonicalize those. Can you list others? We can make some kind of map if there are a lot, but each case requires its own constant adjustment, so we'd end up with a switch I think.


https://reviews.llvm.org/D48485