[AMDGPU] Make S_WAIT_EVENT a scheduling boundary

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -4214,6 +4214,13 @@ bool SIInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
   if (MI.getOpcode() == AMDGPU::SCHED_BARRIER && MI.getOperand(0).getImm() == 0)
     return true;
 
+  // The scheduler does not understand what kind of external events this
+  // instruction waits for, so cannot do a good job of scheduling it. Making it
+  // a boundary allows front ends to insert it at an appropriate place without
+  // the scheduler arbitrarily moving it.
+  if (MI.getOpcode() == AMDGPU::S_WAIT_EVENT)
+    return true;
+
   // Target-independent instructions do not have an implicit-use of EXEC, even
   // when they operate on VGPRs. Treating EXEC modifications as scheduling
   // boundaries prevents incorrect movements of such instructions.

llvm/test/CodeGen/AMDGPU/sched-wait-event.ll

@@ -22,18 +22,18 @@ define amdgpu_ps void @test_wait_event(i32 inreg %arg, float %arg1, float %arg2,
 ; GFX12-NEXT:    s_mov_b32 exec_lo, s0
 ; GFX12-NEXT:    v_interp_p10_f32 v4, v2, v1, v2 wait_exp:1
 ; GFX12-NEXT:    v_interp_p10_f32 v1, v3, v1, v3 wait_exp:0
-; GFX12-NEXT:    s_wait_event 0x2
-; GFX12-NEXT:    v_mov_b32_e32 v8, 0
 ; GFX12-NEXT:    v_interp_p2_f32 v4, v2, v0, v4 wait_exp:7
 ; GFX12-NEXT:    v_interp_p2_f32 v0, v3, v0, v1 wait_exp:7
 ; GFX12-NEXT:    v_mul_f32_e32 v1, 0x44800000, v4
 ; GFX12-NEXT:    v_mul_f32_e32 v0, 0x44800000, v0
 ; GFX12-NEXT:    v_cvt_i32_f32_e32 v1, v1
 ; GFX12-NEXT:    v_cvt_i32_f32_e32 v0, v0
+; GFX12-NEXT:    s_wait_event 0x2
 ; GFX12-NEXT:    image_load v[4:7], [v1, v0], s[4:11] dmask:0xf dim:SQ_RSRC_IMG_2D
 ; GFX12-NEXT:    s_wait_loadcnt 0x0
-; GFX12-NEXT:    v_dual_mul_f32 v7, 0.5, v7 :: v_dual_mul_f32 v6, 0.5, v6
-; GFX12-NEXT:    v_dual_mul_f32 v5, 0.5, v5 :: v_dual_mul_f32 v4, 0.5, v4
+; GFX12-NEXT:    v_dual_mov_b32 v8, 0 :: v_dual_mul_f32 v7, 0.5, v7
+; GFX12-NEXT:    v_dual_mul_f32 v6, 0.5, v6 :: v_dual_mul_f32 v5, 0.5, v5
+; GFX12-NEXT:    v_mul_f32_e32 v4, 0.5, v4
 ; GFX12-NEXT:    image_store v[4:7], [v1, v0], s[4:11] dmask:0xf dim:SQ_RSRC_IMG_2D
 ; GFX12-NEXT:    s_wait_storecnt 0x0
 ; GFX12-NEXT:    export mrt0 v8, v8, v8, v8 done