[PATCH] D107711: [RISCV] Optimize (add (mul x, c0), c1)

[Ben Shi via Phabricator via llvm-commits]

benshi001 added inline comments.


================
Comment at: llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp:537
+    SDNode *NM =
+        CurDAG->getMachineNode(RISCV::MUL, DL, VT, SDValue(NA, 0),
+                               N0->getOperand(1));
----------------
craig.topper wrote:
> What about all the optimizations we have for mul by constant using SHXADD? Won't this miss out on those?
I do not think it will affect the optimization with SHxADD.

For the ones such as
(mul 11/13/25/41/73/37/21), (mul (3/5/9)*power_of_2), (mul power_of_2 + (2/4/8)), those are pure mul without add involved, so they won't be affected.

For the rules such as
(x + y * 4) -> (SH2ADD y, x)
(x + y * 20) -> (SH2ADD (SH2ADD x, x), y)
.......

My patch still generate better code.

c1 + x * 72 (c1 is a non-simm12 constant)

before current patch
```
lui Ry, higher-bits of c1
addi Ry, Ry, lower-12-bits of c1
sh3add Rz, x, x
sh3add Rz, Rz, Ry
```

after my patch
```
addi Ry, x, c1/72
sh3add Ry, Ry, Ry
sll Ry, Ry, 3
```

I will add those cases to the test file.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D107711/new/

https://reviews.llvm.org/D107711