[PATCH] D153106: [CSKY] Optimize multiplication with immediates

Mon Jun 19 02:24:17 PDT 2023

benshi001 added inline comments.

================
Comment at: llvm/lib/Target/CSKY/CSKYISelLowering.cpp:1394
+    if ((Imm + 1).isPowerOf2() || (Imm - 1).isPowerOf2() ||
+        (1 - Imm).isPowerOf2())
+      return true;
----------------
benshi001 wrote:
> zixuan-wu wrote:
> > Does `(-1 - Imm).isPowerOf2()` also make sense?
> `(-1 - Imm).isPowerOf2()` will generate more instructions, I am not sure this is a win, though the multiplication is eliminated. So I left `(-1 - Imm)` unchanged. Maybe we can decide this case later.
Especially, for example,

`a * -4097` will be generated to

```
lsli16, a1, a0, 12
addu16 a1, a0
movi16 a0, 0
subu16 a0, a1
```

An extra 'movi16` is needed. 

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153106/new/

https://reviews.llvm.org/D153106