[clang] [lld] [llvm] [LLVM][WebAssembly] Implement branch hinting proposal (PR #146230)

[Lukas Döllerer via llvm-commits]

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@@ -697,6 +738,34 @@ void WebAssemblyAsmPrinter::emitInstruction(const MachineInstr *MI) {
     WebAssemblyMCInstLower MCInstLowering(OutContext, *this);
     MCInst TmpInst;
     MCInstLowering.lower(MI, TmpInst);
+    if (Subtarget->hasBranchHinting() &&
+        MI->getOpcode() == WebAssembly::BR_IF && MFI &&
+        MFI->BranchProbabilities.contains(MI)) {
+      MCSymbol *BrIfSym = OutContext.createTempSymbol();
+      OutStreamer->emitLabel(BrIfSym);
+
+      constexpr uint8_t HintLikely = 0x01;
+      constexpr uint8_t HintUnlikely = 0x00;
+      const BranchProbability &Prob = MFI->BranchProbabilities[MI];
+      uint8_t HintValue;
+      if (Prob > BranchProbability::getRaw(WasmHighBranchProb.getValue()))
+        HintValue = HintLikely;
+      else if (Prob <= BranchProbability::getRaw(WasmLowBranchProb.getValue()))
+        HintValue = HintUnlikely;
+      else
+        goto emit; // Don't emit branch hint between thresholds
+
+      // we know that we only emit branch hints for internal functions,
+      // therefore we can directly cast and don't need getMCSymbolForFunction
+      MCSymbol *FuncSym = cast<MCSymbolWasm>(getSymbol(&MF->getFunction()));
+      uint32_t LocalFuncIdx = MF->getFunctionNumber();
+      if (branchHints.size() <= LocalFuncIdx) {
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Lukasdoe wrote:

Please check whether I understood correctly what you meant here. The resizing is now done once for each function.

https://github.com/llvm/llvm-project/pull/146230