[PATCH] D88591: [WebAssembly] Emulate v128.const efficiently

[Hubert Tong via Phabricator via llvm-commits]

hubert.reinterpretcast added inline comments.


================
Comment at: llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp:1631
+    Result = DAG.getSplatBuildVector(MVT::v2i64, DL,
+                                     DAG.getConstant(Splatted, DL, MVT::i64));
+    if (!FirstHalfSufficient && !SecondHalfSufficient && !CombinedSufficient) {
----------------
dweber wrote:
> hubert.reinterpretcast wrote:
> > `Splatted` represents a different value on big endian systems at this point.
> @hubert.reinterpretcast If you remove the byte_swap function all together, do you still get the same issue? 
I mean, from code inspection, without needing to assume anything weird on the original `byte_swap`.

Using half the width as an example (4 elements of 16 bits):
Values:
```
0x1234 0x5678 0x1234 0x5678
```

Resulting bytes in storage:
```
0x34 0x12 0x78 0x56 // 0x34 0x12 0x78 0x56
0xff 0xff 0xff 0xff // 0xff 0xff 0xff 0xff
```

Request constant from value with bytes:
```
0x34 0x12 0x78 0x56
```
i.e., on little endian systems, request constant with value:
```
0x56781234
```
or, on big endian systems, request constant with value:
```
0x34127856
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D88591/new/

https://reviews.llvm.org/D88591