[llvm] AMDGPU: Figure out required AGPR count for inline asm (PR #150910)

[Fabian Ritter via llvm-commits]

================
@@ -1211,16 +1211,66 @@ AAAMDWavesPerEU &AAAMDWavesPerEU::createForPosition(const IRPosition &IRP,
   llvm_unreachable("AAAMDWavesPerEU is only valid for function position");
 }
 
-static bool inlineAsmUsesAGPRs(const InlineAsm *IA) {
-  for (const auto &CI : IA->ParseConstraints()) {
+/// Compute the minimum number of AGPRs required to allocate the inline asm.
+static unsigned inlineAsmGetNumRequiredAGPRs(const InlineAsm *IA,
+                                             const CallBase &Call) {
+  unsigned ArgNo = 0;
+  unsigned ResNo = 0;
+  unsigned AGPRDefCount = 0;
+  unsigned AGPRUseCount = 0;
+  unsigned MaxPhysReg = 0;
+  const DataLayout &DL = Call.getFunction()->getParent()->getDataLayout();
+
+  for (const InlineAsm::ConstraintInfo &CI : IA->ParseConstraints()) {
+    Type *Ty = nullptr;
+    switch (CI.Type) {
+    case InlineAsm::isOutput: {
+      Ty = Call.getType();
+      if (auto *STy = dyn_cast<StructType>(Ty))
+        Ty = STy->getElementType(ResNo);
+      ++ResNo;
+      break;
+    }
+    case InlineAsm::isInput: {
+      Ty = Call.getArgOperand(ArgNo++)->getType();
+      break;
+    }
+    case InlineAsm::isLabel:
+      continue;
+    case InlineAsm::isClobber:
+      // Parse the physical register reference.
+      break;
+    }
+
     for (StringRef Code : CI.Codes) {
-      Code.consume_front("{");
-      if (Code.starts_with("a"))
-        return true;
+      unsigned RegCount = 0;
+
+      if (Code.starts_with("a")) {
+        // Virtual register, compute number of registers based on the type.
+        //
+        // We ought to be going through TargetLowering to get the number of
+        // registers, but we should avoid the dependence on CodeGen here.
+        RegCount = divideCeil(DL.getTypeSizeInBits(Ty), 32);
+      } else {
+        // Physical register reference
+        auto [Kind, RegIdx, NumRegs] = AMDGPU::parseAsmConstraintPhysReg(Code);
+        if (Kind == 'a') {
+          RegCount = NumRegs;
+          MaxPhysReg = std::max(MaxPhysReg, std::min(RegIdx + NumRegs, 256u));
+        }
+      }
+
+      if (CI.Type == InlineAsm::isOutput) {
+        AGPRDefCount += RegCount;
+        if (CI.isEarlyClobber)
+          AGPRUseCount += RegCount;
+      } else
+        AGPRUseCount += RegCount;
     }
   }
 
-  return false;
+  unsigned MaxVirtReg = std::max(AGPRUseCount, AGPRDefCount);
+  return std::min(std::max(MaxVirtReg, MaxPhysReg), 256u);
----------------
ritter-x2a wrote:

I see, I missed that RegCount is also set in the physical case.
If the expected answer for this example is 8 (and not 9), even though this can only be allocated using `a[1:4]` and `a[5:8]` (or higher indices; because the virtual registers are also contiguous), why do you need to check `MaxPhysReg`? It seems odd to me that this would report 8, but inline ASM that just has a physical register constraint for `a[5:8]` would count as 9.

https://github.com/llvm/llvm-project/pull/150910