[llvm] [AMDGPU] Rework dot4 signedness checks (PR #68757)

[Matt Arsenault via llvm-commits]

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@@ -13056,32 +13023,74 @@ static bool isMul(const SDValue Op) {
           Opcode == AMDGPUISD::MUL_I24);
 }
 
-static std::optional<bool> checkSignedness(const SDValue &N,
-                                           ByteProvider<SDValue> &Src0,
-                                           ByteProvider<SDValue> &Src1) {
+static std::optional<bool>
+checkSignedness(const SDValue &N, ByteProvider<SDValue> &Src0,
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arsenm wrote:

Can you rename this function to show it's for multiplies? As far as I can tell this is just performing sign bit logic on a multiply. As in, you're effectively reproducing what computeKnownBits is doing for a multiply. Is it possible to just check the result of computeKnownBits on the top multiply?

https://github.com/llvm/llvm-project/pull/68757