[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:
> dweber wrote:
> > dweber wrote:
> > > hubert.reinterpretcast wrote:
> > > > 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
> > > > ```
> > > @hubert.reinterpretcast the reason I ask is because webassembly assumes little endian. On big endian machines, the constant could have been converted before we do anything here -- especially since the developer using emscripten is taught there is no other byte order but little endian. If that's the case, the byte swap could be creating the problem. 
> > > 
> > > 
> > (the above byte_swap on a little endian machine will always produce the original parameter without fail).
> Two byte swaps would definitely be safe though.
The first `byte_swap` is necessary. The following makes the case pass:
```
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
index 8474e50..5a94041 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -1586,8 +1586,10 @@ SDValue WebAssemblyTargetLowering::LowerBUILD_VECTOR(SDValue Op,
     // we can construct the 128-bit constant from a pair of i64s using a splat
     // followed by at most one i64x2.replace_lane. Also keep track of the lanes
     // that actually matter so we can avoid the replace_lane in more cases.
-    std::array<uint64_t, 2> I64s({0, 0});
-    std::array<uint64_t, 2> ConstLaneMasks({0, 0});
+    using llvm::support::ulittle64_t;
+    const ulittle64_t ulittle64_zero(0);
+    std::array<ulittle64_t, 2> I64s({ulittle64_zero, ulittle64_zero});
+    std::array<ulittle64_t, 2> ConstLaneMasks({ulittle64_zero, ulittle64_zero});
     uint8_t *I64Bytes = reinterpret_cast<uint8_t *>(I64s.data());
     uint8_t *MaskBytes = reinterpret_cast<uint8_t *>(ConstLaneMasks.data());
     unsigned I = 0;
```


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  https://reviews.llvm.org/D88591/new/

https://reviews.llvm.org/D88591