[AMDGPU] Add intrinsic exposing s_alloc_vgpr

llvm/include/llvm/IR/IntrinsicsAMDGPU.td

@@ -391,6 +391,16 @@ def int_amdgcn_s_wait_loadcnt        : AMDGPUWaitIntrinsic;
 def int_amdgcn_s_wait_samplecnt      : AMDGPUWaitIntrinsic;
 def int_amdgcn_s_wait_storecnt       : AMDGPUWaitIntrinsic;
 
+// Request the hardware to allocate the given number of VGPRs. The actual number
+// of allocated VGPRs may be rounded up to match hardware block boundaries.
+// It is the responsibility of the calling code to ensure it does not allocate
+// below the VGPR requirements of the current shader.
+def int_amdgcn_s_alloc_vgpr :
+  DefaultAttrsIntrinsic<
+    [llvm_i1_ty], // Returns true if the allocation succeeded, false otherwise.
+    [llvm_i32_ty], // The number of VGPRs to allocate.
+    [NoUndef<RetIndex>, IntrNoMem, IntrHasSideEffects, IntrConvergent]>;
+
 def int_amdgcn_div_scale : DefaultAttrsIntrinsic<
   // 1st parameter: Numerator
   // 2nd parameter: Denominator

llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp

@@ -2331,6 +2331,22 @@ bool AMDGPUInstructionSelector::selectG_INTRINSIC_W_SIDE_EFFECTS(
   case Intrinsic::amdgcn_ds_bvh_stack_push8_pop1_rtn:
   case Intrinsic::amdgcn_ds_bvh_stack_push8_pop2_rtn:
     return selectDSBvhStackIntrinsic(I);
+  case Intrinsic::amdgcn_s_alloc_vgpr: {
+    // S_ALLOC_VGPR doesn't have a destination register, it just implicitly sets
+    // SCC. We then need to COPY it into the result vreg.
+    MachineBasicBlock *MBB = I.getParent();
+    const DebugLoc &DL = I.getDebugLoc();
+
+    Register ResReg = I.getOperand(0).getReg();
+
+    MachineInstr *AllocMI = BuildMI(*MBB, &I, DL, TII.get(AMDGPU::S_ALLOC_VGPR))
+                                .add(I.getOperand(2));
+    (void)BuildMI(*MBB, &I, DL, TII.get(AMDGPU::COPY), ResReg)
+        .addReg(AMDGPU::SCC);
+    I.eraseFromParent();
+    return constrainSelectedInstRegOperands(*AllocMI, TII, TRI, RBI) &&
+           RBI.constrainGenericRegister(ResReg, AMDGPU::SReg_32RegClass, *MRI);
+  }
   case Intrinsic::amdgcn_s_barrier_init:
   case Intrinsic::amdgcn_s_barrier_signal_var:
     return selectNamedBarrierInit(I, IntrinsicID);

llvm/lib/Target/AMDGPU/AMDGPURegisterBankInfo.cpp

@@ -5359,6 +5359,10 @@ AMDGPURegisterBankInfo::getInstrMapping(const MachineInstr &MI) const {
       OpdsMapping[6] = AMDGPU::getValueMapping(AMDGPU::VGPRRegBankID, 32);
       OpdsMapping[8] = getSGPROpMapping(MI.getOperand(8).getReg(), MRI, *TRI);
       break;
+    case Intrinsic::amdgcn_s_alloc_vgpr:
+      OpdsMapping[0] = AMDGPU::getValueMapping(AMDGPU::SGPRRegBankID, 1);
+      OpdsMapping[2] = AMDGPU::getValueMapping(AMDGPU::SGPRRegBankID, 32);
+      break;
     case Intrinsic::amdgcn_s_sendmsg:
     case Intrinsic::amdgcn_s_sendmsghalt: {
       // This must be an SGPR, but accept a VGPR.

llvm/lib/Target/AMDGPU/AMDGPUSearchableTables.td

@@ -409,6 +409,7 @@ def : AlwaysUniform<int_amdgcn_cluster_workgroup_max_flat_id>;
 def : AlwaysUniform<int_amdgcn_workgroup_id_x>;
 def : AlwaysUniform<int_amdgcn_workgroup_id_y>;
 def : AlwaysUniform<int_amdgcn_workgroup_id_z>;
+def : AlwaysUniform<int_amdgcn_s_alloc_vgpr>;
 def : AlwaysUniform<int_amdgcn_s_getpc>;
 def : AlwaysUniform<int_amdgcn_s_getreg>;
 def : AlwaysUniform<int_amdgcn_s_memrealtime>;

llvm/lib/Target/AMDGPU/SOPInstructions.td

@@ -433,8 +433,10 @@ let SubtargetPredicate = isGFX11Plus in {
 } // End SubtargetPredicate = isGFX11Plus
 
 let SubtargetPredicate = isGFX12Plus in {
-  let hasSideEffects = 1, Defs = [SCC] in {
-    def S_ALLOC_VGPR : SOP1_0_32 <"s_alloc_vgpr">;
+  let hasSideEffects = 1, isConvergent = 1, Defs = [SCC] in {
+    def S_ALLOC_VGPR : SOP1_0_32 <"s_alloc_vgpr",
+      [(set SCC, (int_amdgcn_s_alloc_vgpr SSrc_b32:$src0))]
+    >;
   }
 } // End SubtargetPredicate = isGFX12Plus
 

llvm/test/Analysis/UniformityAnalysis/AMDGPU/always_uniform.ll

@@ -183,6 +183,15 @@ define void @cluster_workgroup_max_flat_id(ptr addrspace(1) inreg %out) {
   ret void
 }
 
+; CHECK-LABEL: for function 's_alloc_vgpr':
+; CHECK: ALL VALUES UNIFORM
+define void @s_alloc_vgpr(i32 inreg %n, ptr addrspace(1) inreg %out) {
+  %scc = call i1 @llvm.amdgcn.s.alloc.vgpr(i32 %n)
+  %sel = select i1 %scc, i32 1, i32 0
+  store i32 %sel, ptr addrspace(1) %out
+  ret void
+}
+
 ; CHECK-LABEL: for function 's_memtime':
 ; CHECK: ALL VALUES UNIFORM
 define void @s_memtime(ptr addrspace(1) inreg %out) {

llvm/test/CodeGen/AMDGPU/intrinsic-amdgcn-s-alloc-vgpr.ll

@@ -0,0 +1,59 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -global-isel=1 -mtriple=amdgcn--amdpal -mcpu=gfx1250 < %s | FileCheck %s --check-prefix=GISEL
+; RUN: llc -global-isel=0 -mtriple=amdgcn--amdpal -mcpu=gfx1250 < %s | FileCheck %s --check-prefix=DAGISEL
+
+declare i1 @llvm.amdgcn.s.alloc.vgpr(i32)
+
+define amdgpu_cs void @test_alloc_vreg_const(ptr addrspace(1) %out) #0 {
+; GISEL-LABEL: test_alloc_vreg_const:
+; GISEL:       ; %bb.0: ; %entry
+; GISEL-NEXT:    s_alloc_vgpr 45
+; GISEL-NEXT:    s_cselect_b32 s0, 1, 0
+; GISEL-NEXT:    s_delay_alu instid0(SALU_CYCLE_1) | instskip(NEXT) | instid1(SALU_CYCLE_1)
+; GISEL-NEXT:    s_and_b32 s0, s0, 1
+; GISEL-NEXT:    v_mov_b32_e32 v2, s0
+; GISEL-NEXT:    global_store_b32 v[0:1], v2, off
+; GISEL-NEXT:    s_endpgm
+;
+; DAGISEL-LABEL: test_alloc_vreg_const:
+; DAGISEL:       ; %bb.0: ; %entry
+; DAGISEL-NEXT:    s_alloc_vgpr 45
+; DAGISEL-NEXT:    s_cselect_b32 s0, -1, 0
+; DAGISEL-NEXT:    s_delay_alu instid0(SALU_CYCLE_1)
+; DAGISEL-NEXT:    v_cndmask_b32_e64 v2, 0, 1, s0
+; DAGISEL-NEXT:    global_store_b32 v[0:1], v2, off
+; DAGISEL-NEXT:    s_endpgm
+entry:
+  %scc = call i1 @llvm.amdgcn.s.alloc.vgpr(i32 45)
+  %sel = select i1 %scc, i32 1, i32 0
+  store i32 %sel, ptr addrspace(1) %out
+  ret void
+}
+
+define amdgpu_cs void @test_alloc_vreg_var(i32 inreg %n, ptr addrspace(1) %out) #0 {
+; GISEL-LABEL: test_alloc_vreg_var:
+; GISEL:       ; %bb.0: ; %entry
+; GISEL-NEXT:    s_alloc_vgpr s0
+; GISEL-NEXT:    s_cselect_b32 s0, 1, 0
+; GISEL-NEXT:    s_delay_alu instid0(SALU_CYCLE_1) | instskip(NEXT) | instid1(SALU_CYCLE_1)
+; GISEL-NEXT:    s_and_b32 s0, s0, 1
+; GISEL-NEXT:    v_mov_b32_e32 v2, s0
+; GISEL-NEXT:    global_store_b32 v[0:1], v2, off
+; GISEL-NEXT:    s_endpgm
+;
+; DAGISEL-LABEL: test_alloc_vreg_var:
+; DAGISEL:       ; %bb.0: ; %entry
+; DAGISEL-NEXT:    s_alloc_vgpr s0
+; DAGISEL-NEXT:    s_cselect_b32 s0, -1, 0
+; DAGISEL-NEXT:    s_delay_alu instid0(SALU_CYCLE_1)
+; DAGISEL-NEXT:    v_cndmask_b32_e64 v2, 0, 1, s0
+; DAGISEL-NEXT:    global_store_b32 v[0:1], v2, off
+; DAGISEL-NEXT:    s_endpgm
+entry:
+  %scc = call i1 @llvm.amdgcn.s.alloc.vgpr(i32 %n)
+  %sel = select i1 %scc, i32 1, i32 0
+  store i32 %sel, ptr addrspace(1) %out
+  ret void
+}
+
+attributes #0 = { "amdgpu-dynamic-vgpr-block-sze" = "16" }