[llvm] AMDGPU gfx12: Add _dvgpr$ symbols for dynamic VGPRs (PR #148251)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Tim Renouf (trenouf)

<details>
<summary>Changes</summary>

For each function with the AMDGPU_CS_Chain calling convention, with dynamic VGPRs enabled, add a _dvgpr$ symbol, with the value of the function symbol, plus an offset encoding one less than the number of VGPR blocks used by the function (16 VGPRs per block, no more than 128) in bits 5..3 of the symbol value. This is used by a front-end to have functions that are chained rather than called, and a dispatcher that dynamically resizes the VGPR count before dispatching to a function.

---
Full diff: https://github.com/llvm/llvm-project/pull/148251.diff


2 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp (+26) 
- (added) llvm/test/CodeGen/AMDGPU/dvgpr_sym.ll (+12) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp b/llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp
index 749b9efc81378..00ed5f57967ce 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp
@@ -194,6 +194,32 @@ void AMDGPUAsmPrinter::emitFunctionBodyStart() {
     return;
   }
 
+  if (STM.isDynamicVGPREnabled() &&
+      MF->getFunction().getCallingConv() == CallingConv::AMDGPU_CS_Chain) {
+    // Add a _dvgpr$ symbol, with the value of the function symbol, plus an
+    // offset encoding one less than the number of VGPR blocks used by the
+    // function (16 VGPRs per block, no more than 128) in bits 5..3 of the
+    // symbol value. This is used by a front-end to have functions that are
+    // chained rather than called, and a dispatcher that dynamically resizes
+    // the VGPR count before dispatching to a function.
+    ResourceUsage = &getAnalysis<AMDGPUResourceUsageAnalysis>();
+    const AMDGPUResourceUsageAnalysis::SIFunctionResourceInfo &Info =
+        ResourceUsage->getResourceInfo();
+    MCContext &Ctx = MF->getContext();
+    unsigned EncodedNumVGPRs = (Info.NumVGPR - 1) >> 1 & 0x38;
+    MCSymbol *CurPCSym = Ctx.createTempSymbol();
+    OutStreamer->emitLabel(CurPCSym);
+    const MCExpr *DVgprFuncVal = MCBinaryExpr::createAdd(
+        MCSymbolRefExpr::create(CurPCSym, MCSymbolRefExpr::VK_None, Ctx),
+        MCConstantExpr::create(EncodedNumVGPRs, Ctx), Ctx);
+    MCSymbol *DVgprFuncSym =
+        Ctx.getOrCreateSymbol(Twine("_dvgpr$") + MF->getFunction().getName());
+    OutStreamer->emitAssignment(DVgprFuncSym, DVgprFuncVal);
+    cast<MCSymbolELF>(DVgprFuncSym)
+        ->setBinding(
+            cast<MCSymbolELF>(getSymbol(&MF->getFunction()))->getBinding());
+  }
+
   if (!MFI.isEntryFunction())
     return;
 
diff --git a/llvm/test/CodeGen/AMDGPU/dvgpr_sym.ll b/llvm/test/CodeGen/AMDGPU/dvgpr_sym.ll
new file mode 100644
index 0000000000000..992963d304ead
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/dvgpr_sym.ll
@@ -0,0 +1,12 @@
+; Test generation of _dvgpr$ symbol for an amdgpu_cs_chain function with +dynamic-vgpr.
+
+; RUN: llc -mtriple=amdgcn-amd-amdpal -mcpu=gfx1200 -asm-verbose=0 < %s | FileCheck -check-prefixes=DVGPR %s
+
+; DVGPR-LABEL: func:
+; DVGPR: .Ltmp0:
+; DVGPR: .set _dvgpr$func, .Ltmp0+{{[0-9]+}}
+
+define amdgpu_cs_chain void @func() #0 {
+  ret void
+}
+attributes #0 = { "target-features"="+dynamic-vgpr" }

``````````

</details>


https://github.com/llvm/llvm-project/pull/148251