[all-commits] [llvm/llvm-project] e9fd88: [DAGCombiner] Add decomposition patterns for Mul-b...

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  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: e9fd8823baf508ff4e96cff9c24bc301ef4d100e
      https://github.com/llvm/llvm-project/commit/e9fd8823baf508ff4e96cff9c24bc301ef4d100e
  Author: Esme-Yi <esme.yi at ibm.com>
  Date:   2020-10-09 (Fri, 09 Oct 2020)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    M llvm/lib/Target/PowerPC/PPCISelLowering.cpp
    M llvm/lib/Target/PowerPC/PPCISelLowering.h
    M llvm/test/CodeGen/PowerPC/mulli.ll

  Log Message:
  -----------
  [DAGCombiner] Add decomposition patterns for Mul-by-Imm.

Summary: This patch is derived from D87384.
In this patch we expand the existing decomposition of mul-by-constant to be more general by implementing 2 patterns:
```
  mul x, (2^N + 2^M) --> (add (shl x, N), (shl x, M))
  mul x, (2^N - 2^M) --> (sub (shl x, N), (shl x, M))
```
The conversion will be trigged if the multiplier is a big constant that the target can't use a single multiplication instruction to handle. This is controlled by the hook `decomposeMulByConstant`.
More over, the conversion benefits from an ILP improvement since the instructions are independent. A case with the sequence like following also gets benefit since a shift instruction is saved.

```
*res1 = a * 0x8800;
*res2 = a * 0x8080;
```

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D88201