[llvm] [SPIRV] Implement translation for llvm.modf.* intrinsics (PR #147556)

[Marcos Maronas via llvm-commits]

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@@ -3990,6 +3995,77 @@ bool SPIRVInstructionSelector::selectLog10(Register ResVReg,
                        .constrainAllUses(TII, TRI, RBI);
 }
 
+bool SPIRVInstructionSelector::selectModf(Register ResVReg,
+                                          const SPIRVType *ResType,
+                                          MachineInstr &I) const {
+  // llvm.modf has a single arg --the number to be decomposed-- and returns a
+  // struct { restype, restype }, while OpenCLLIB::modf has two args --the
+  // number to be decomposed and a pointer--, returns the fractional part and
+  // the integral part is stored in the pointer argument. Therefore, we can't
+  // use directly the OpenCLLIB::modf intrinsic. However, we can do some
+  // scaffolding to make it work. The idea is to create an alloca instruction
+  // to get a ptr, pass this ptr to OpenCL::modf, and then load the value
+  // from this ptr to place it in the struct. llvm.modf returns the fractional
+  // part as the first element of the result, and the integral part as the
+  // second element of the result.
+
+  // At this point, the return type is not a struct anymore, but rather two
+  // independent elements of SPIRVResType. We can get each independent element
+  // from I.getDefs() or I.getOperands().
+  ExtInstList ExtInsts = {{SPIRV::InstructionSet::OpenCL_std, CL::modf},
+                          {SPIRV::InstructionSet::GLSL_std_450, GL::Modf}};
+  for (const auto &Ex : ExtInsts) {
+    SPIRV::InstructionSet::InstructionSet Set = Ex.first;
+    uint32_t Opcode = Ex.second;
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maarquitos14 wrote:

Honestly, I just copied the structure from `SPIRVInstructionSelector::selectExtInst` and adapted it for this particular case, but I do agree it seems convoluted. Also, if `GL::Modf` is deprecated there's no reason to keep this. I'll simplify it, thanks for catching this.

https://github.com/llvm/llvm-project/pull/147556