vg0204 · Nov 6, 2026
diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
@@ -5218,8 +5218,19 @@ bool SIInstrInfo::isMaskedByExec(Register Reg, const MachineInstr &Use,
     return true;
   if ((Opc == LMC.OrOpc || Opc == LMC.XorOpc) && Recurse(1) && Recurse(2))
     return true;
-  // TODO: Sometimes we encounter "reg = S_CSELECT -1, 0". If Reg has no other
-  // uses this could be optimized to "reg = S_CSELECT $exec, 0".
+  // S_CSELECT x, 0 and S_CSELECT 0, x produce either x or 0. 0 has no bits set
+  // for inactive lanes, so the result is masked iff x is. An immediate x (e.g.
+  // -1) is rejected by Recurse, since it may set bits for inactive lanes.
+  if (Opc == LMC.CSelectOpc) {
+    std::optional<int64_t> FalseImm =
+        getImmOrMaterializedImm(MRI, Def->getOperand(2));
+    if (FalseImm && *FalseImm == 0)
+      return Recurse(1);
+    std::optional<int64_t> TrueImm =
+        getImmOrMaterializedImm(MRI, Def->getOperand(1));
+    if (TrueImm && *TrueImm == 0)
+      return Recurse(2);
+  }
 
   return false;
 }

diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-divergent-i1-phis-no-lane-mask-merging.ll b/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-divergent-i1-phis-no-lane-mask-merging.ll
@@ -112,11 +112,10 @@ define void @divergent_i1_phi_used_inside_loop(float %val, ptr %addr) {
 ; GFX10-NEXT:    s_cselect_b32 s8, exec_lo, 0
 ; GFX10-NEXT:    v_cmp_gt_f32_e32 vcc_lo, v3, v0
 ; GFX10-NEXT:    s_andn2_b32 s7, s7, exec_lo
-; GFX10-NEXT:    s_and_b32 s8, exec_lo, s8
 ; GFX10-NEXT:    s_xor_b32 s5, s5, 1
 ; GFX10-NEXT:    s_add_i32 s6, s6, 1
-; GFX10-NEXT:    s_or_b32 s4, vcc_lo, s4
 ; GFX10-NEXT:    s_or_b32 s7, s7, s8
+; GFX10-NEXT:    s_or_b32 s4, vcc_lo, s4
 ; GFX10-NEXT:    s_andn2_b32 exec_lo, exec_lo, s4
 ; GFX10-NEXT:    s_cbranch_execnz .LBB2_1
 ; GFX10-NEXT:  ; %bb.2: ; %exit

diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-divergent-i1-used-outside-loop.ll b/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-divergent-i1-used-outside-loop.ll
@@ -552,13 +552,12 @@ define amdgpu_cs void @loop_with_1break(ptr addrspace(1) %x, ptr addrspace(1) %a
 ; GFX10-NEXT:  ; %bb.3: ; %loop.body
 ; GFX10-NEXT:    ; in Loop: Header=BB7_2 Depth=1
 ; GFX10-NEXT:    global_load_dword v6, v[0:1], off
-; GFX10-NEXT:    s_add_i32 s1, s1, 1
 ; GFX10-NEXT:    v_add_co_u32 v2, vcc_lo, v2, 4
-; GFX10-NEXT:    s_cmpk_lt_u32 s1, 0x64
+; GFX10-NEXT:    s_add_i32 s1, s1, 1
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v3, vcc_lo, 0, v3, vcc_lo
+; GFX10-NEXT:    s_cmpk_lt_u32 s1, 0x64
 ; GFX10-NEXT:    s_cselect_b32 s6, exec_lo, 0
 ; GFX10-NEXT:    s_andn2_b32 s4, s4, exec_lo
-; GFX10-NEXT:    s_and_b32 s6, exec_lo, s6
 ; GFX10-NEXT:    s_andn2_b32 s3, s3, exec_lo
 ; GFX10-NEXT:    s_or_b32 s4, s4, s6
 ; GFX10-NEXT:    s_waitcnt vmcnt(0)

diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-structurizer.ll b/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-structurizer.ll
@@ -126,13 +126,12 @@ define amdgpu_cs void @loop_with_1break(ptr addrspace(1) %x, ptr addrspace(1) %a
 ; GFX10-NEXT:  ; %bb.3: ; %loop.body
 ; GFX10-NEXT:    ; in Loop: Header=BB2_2 Depth=1
 ; GFX10-NEXT:    global_load_dword v4, v[0:1], off
-; GFX10-NEXT:    s_add_i32 s0, s0, 1
 ; GFX10-NEXT:    v_add_co_u32 v2, vcc_lo, v2, 4
-; GFX10-NEXT:    s_cmpk_lt_u32 s0, 0x64
+; GFX10-NEXT:    s_add_i32 s0, s0, 1
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v3, vcc_lo, 0, v3, vcc_lo
+; GFX10-NEXT:    s_cmpk_lt_u32 s0, 0x64
 ; GFX10-NEXT:    s_cselect_b32 s4, exec_lo, 0
 ; GFX10-NEXT:    s_andn2_b32 s2, s2, exec_lo
-; GFX10-NEXT:    s_and_b32 s4, exec_lo, s4
 ; GFX10-NEXT:    s_or_b32 s2, s2, s4
 ; GFX10-NEXT:    s_waitcnt vmcnt(0)
 ; GFX10-NEXT:    v_add_nc_u32_e32 v4, 1, v4
@@ -207,13 +206,12 @@ define amdgpu_cs void @loop_with_2breaks(ptr addrspace(1) %x, ptr addrspace(1) %
 ; GFX10-NEXT:    global_load_dword v6, v[0:1], off
 ; GFX10-NEXT:    v_add_co_u32 v2, vcc_lo, v2, 4
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v3, vcc_lo, 0, v3, vcc_lo
-; GFX10-NEXT:    s_add_i32 s0, s0, 1
 ; GFX10-NEXT:    v_add_co_u32 v4, vcc_lo, v4, 4
-; GFX10-NEXT:    s_cmpk_lt_u32 s0, 0x64
+; GFX10-NEXT:    s_add_i32 s0, s0, 1
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v5, vcc_lo, 0, v5, vcc_lo
+; GFX10-NEXT:    s_cmpk_lt_u32 s0, 0x64
 ; GFX10-NEXT:    s_cselect_b32 s6, exec_lo, 0
 ; GFX10-NEXT:    s_andn2_b32 s4, s4, exec_lo
-; GFX10-NEXT:    s_and_b32 s6, exec_lo, s6
 ; GFX10-NEXT:    s_or_b32 s4, s4, s6
 ; GFX10-NEXT:    s_waitcnt vmcnt(0)
 ; GFX10-NEXT:    v_add_nc_u32_e32 v6, 1, v6
@@ -310,13 +308,12 @@ define amdgpu_cs void @loop_with_3breaks(ptr addrspace(1) %x, ptr addrspace(1) %
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v3, vcc_lo, 0, v3, vcc_lo
 ; GFX10-NEXT:    v_add_co_u32 v4, vcc_lo, v4, 4
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v5, vcc_lo, 0, v5, vcc_lo
-; GFX10-NEXT:    s_add_i32 s0, s0, 1
 ; GFX10-NEXT:    v_add_co_u32 v6, vcc_lo, v6, 4
-; GFX10-NEXT:    s_cmpk_lt_u32 s0, 0x64
+; GFX10-NEXT:    s_add_i32 s0, s0, 1
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v7, vcc_lo, 0, v7, vcc_lo
+; GFX10-NEXT:    s_cmpk_lt_u32 s0, 0x64
 ; GFX10-NEXT:    s_cselect_b32 s8, exec_lo, 0
 ; GFX10-NEXT:    s_andn2_b32 s6, s6, exec_lo
-; GFX10-NEXT:    s_and_b32 s8, exec_lo, s8
 ; GFX10-NEXT:    s_or_b32 s6, s6, s8
 ; GFX10-NEXT:    s_waitcnt vmcnt(0)
 ; GFX10-NEXT:    v_add_nc_u32_e32 v8, 1, v8
@@ -398,13 +395,12 @@ define amdgpu_cs void @loop_with_div_break_with_body(ptr addrspace(1) %x, ptr ad
 ; GFX10-NEXT:  ; %bb.3: ; %loop.body
 ; GFX10-NEXT:    ; in Loop: Header=BB5_2 Depth=1
 ; GFX10-NEXT:    global_load_dword v6, v[0:1], off
-; GFX10-NEXT:    s_add_i32 s1, s1, 1
 ; GFX10-NEXT:    v_add_co_u32 v2, vcc_lo, v2, 4
-; GFX10-NEXT:    s_cmpk_lt_u32 s1, 0x64
+; GFX10-NEXT:    s_add_i32 s1, s1, 1
 ; GFX10-NEXT:    v_add_co_ci_u32_e32 v3, vcc_lo, 0, v3, vcc_lo
+; GFX10-NEXT:    s_cmpk_lt_u32 s1, 0x64
 ; GFX10-NEXT:    s_cselect_b32 s6, exec_lo, 0
 ; GFX10-NEXT:    s_andn2_b32 s4, s4, exec_lo
-; GFX10-NEXT:    s_and_b32 s6, exec_lo, s6
 ; GFX10-NEXT:    s_andn2_b32 s3, s3, exec_lo
 ; GFX10-NEXT:    s_or_b32 s4, s4, s6
 ; GFX10-NEXT:    s_waitcnt vmcnt(0)

diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-temporal-divergent-i1.ll b/llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-temporal-divergent-i1.ll
@@ -17,11 +17,10 @@ define void @temporal_divergent_i1_phi(float %val, ptr %addr) {
 ; GFX10-NEXT:    s_cselect_b32 s8, exec_lo, 0
 ; GFX10-NEXT:    v_cmp_gt_f32_e32 vcc_lo, v3, v0
 ; GFX10-NEXT:    s_andn2_b32 s7, s7, exec_lo
-; GFX10-NEXT:    s_and_b32 s8, exec_lo, s8
 ; GFX10-NEXT:    s_xor_b32 s5, s5, 1
 ; GFX10-NEXT:    s_add_i32 s6, s6, 1
-; GFX10-NEXT:    s_or_b32 s4, vcc_lo, s4
 ; GFX10-NEXT:    s_or_b32 s7, s7, s8
+; GFX10-NEXT:    s_or_b32 s4, vcc_lo, s4
 ; GFX10-NEXT:    s_andn2_b32 exec_lo, exec_lo, s4
 ; GFX10-NEXT:    s_cbranch_execnz .LBB0_1
 ; GFX10-NEXT:  ; %bb.2: ; %exit
@@ -64,11 +63,10 @@ define void @temporal_divergent_i1_non_phi(float %val, ptr %addr) {
 ; GFX10-NEXT:    s_cselect_b32 s8, exec_lo, 0
 ; GFX10-NEXT:    v_cmp_gt_f32_e32 vcc_lo, v3, v0
 ; GFX10-NEXT:    s_andn2_b32 s7, s7, exec_lo
-; GFX10-NEXT:    s_and_b32 s8, exec_lo, s8
 ; GFX10-NEXT:    s_xor_b32 s5, s5, 1
 ; GFX10-NEXT:    s_add_i32 s6, s6, 1
-; GFX10-NEXT:    s_or_b32 s4, vcc_lo, s4
 ; GFX10-NEXT:    s_or_b32 s7, s7, s8
+; GFX10-NEXT:    s_or_b32 s4, vcc_lo, s4
 ; GFX10-NEXT:    s_andn2_b32 exec_lo, exec_lo, s4
 ; GFX10-NEXT:    s_cbranch_execnz .LBB1_1
 ; GFX10-NEXT:  ; %bb.2: ; %exit
@@ -127,7 +125,6 @@ define amdgpu_cs void @loop_with_1break(ptr addrspace(1) %x, i32 %x.size, ptr ad
 ; GFX10-NEXT:    s_cselect_b32 s5, exec_lo, 0
 ; GFX10-NEXT:    s_and_b32 s6, exec_lo, s10
 ; GFX10-NEXT:    s_andn2_b32 s7, s9, exec_lo
-; GFX10-NEXT:    s_and_b32 s5, exec_lo, s5
 ; GFX10-NEXT:    s_or_b32 s8, s6, s8
 ; GFX10-NEXT:    s_or_b32 s9, s7, s5
 ; GFX10-NEXT:    s_waitcnt_depctr depctr_vm_vsrc(0)

diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/llvm.amdgcn.div.fmas.ll b/llvm/test/CodeGen/AMDGPU/GlobalISel/llvm.amdgcn.div.fmas.ll
@@ -1304,7 +1304,6 @@ define amdgpu_kernel void @test_div_fmas_f32_i1_phi_vcc(ptr addrspace(1) %out, [
 ; GFX7-NEXT:    s_cmp_lg_u32 s0, 0
 ; GFX7-NEXT:    s_cselect_b64 s[0:1], exec, 0
 ; GFX7-NEXT:    s_andn2_b64 s[8:9], 0, exec
-; GFX7-NEXT:    s_and_b64 s[0:1], exec, s[0:1]
 ; GFX7-NEXT:    s_or_b64 vcc, s[8:9], s[0:1]
 ; GFX7-NEXT:  .LBB13_2: ; %exit
 ; GFX7-NEXT:    s_or_b64 exec, exec, s[6:7]
@@ -1338,7 +1337,6 @@ define amdgpu_kernel void @test_div_fmas_f32_i1_phi_vcc(ptr addrspace(1) %out, [
 ; GFX8-NEXT:    s_cmp_lg_u32 s0, 0
 ; GFX8-NEXT:    s_cselect_b64 s[0:1], exec, 0
 ; GFX8-NEXT:    s_andn2_b64 s[6:7], 0, exec
-; GFX8-NEXT:    s_and_b64 s[0:1], exec, s[0:1]
 ; GFX8-NEXT:    s_or_b64 vcc, s[6:7], s[0:1]
 ; GFX8-NEXT:  .LBB13_2: ; %exit
 ; GFX8-NEXT:    s_or_b64 exec, exec, s[2:3]
@@ -1371,7 +1369,6 @@ define amdgpu_kernel void @test_div_fmas_f32_i1_phi_vcc(ptr addrspace(1) %out, [
 ; GFX10_W32-NEXT:    s_cmp_lg_u32 s0, 0
 ; GFX10_W32-NEXT:    s_cselect_b32 s0, exec_lo, 0
 ; GFX10_W32-NEXT:    s_andn2_b32 s2, 0, exec_lo
-; GFX10_W32-NEXT:    s_and_b32 s0, exec_lo, s0
 ; GFX10_W32-NEXT:    s_or_b32 vcc_lo, s2, s0
 ; GFX10_W32-NEXT:  .LBB13_2: ; %exit
 ; GFX10_W32-NEXT:    s_or_b32 exec_lo, exec_lo, s1
@@ -1401,7 +1398,6 @@ define amdgpu_kernel void @test_div_fmas_f32_i1_phi_vcc(ptr addrspace(1) %out, [
 ; GFX10_W64-NEXT:    s_cmp_lg_u32 s0, 0
 ; GFX10_W64-NEXT:    s_cselect_b64 s[0:1], exec, 0
 ; GFX10_W64-NEXT:    s_andn2_b64 s[6:7], 0, exec
-; GFX10_W64-NEXT:    s_and_b64 s[0:1], exec, s[0:1]
 ; GFX10_W64-NEXT:    s_or_b64 vcc, s[6:7], s[0:1]
 ; GFX10_W64-NEXT:  .LBB13_2: ; %exit
 ; GFX10_W64-NEXT:    s_or_b64 exec, exec, s[2:3]
@@ -1432,7 +1428,6 @@ define amdgpu_kernel void @test_div_fmas_f32_i1_phi_vcc(ptr addrspace(1) %out, [
 ; GFX11_W32-NEXT:    s_cmp_lg_u32 s0, 0
 ; GFX11_W32-NEXT:    s_cselect_b32 s0, exec_lo, 0
 ; GFX11_W32-NEXT:    s_and_not1_b32 s2, 0, exec_lo
-; GFX11_W32-NEXT:    s_and_b32 s0, exec_lo, s0
 ; GFX11_W32-NEXT:    s_or_b32 vcc_lo, s2, s0
 ; GFX11_W32-NEXT:  .LBB13_2: ; %exit
 ; GFX11_W32-NEXT:    s_or_b32 exec_lo, exec_lo, s1
@@ -1463,7 +1458,6 @@ define amdgpu_kernel void @test_div_fmas_f32_i1_phi_vcc(ptr addrspace(1) %out, [
 ; GFX11_W64-NEXT:    s_cmp_lg_u32 s0, 0
 ; GFX11_W64-NEXT:    s_cselect_b64 s[0:1], exec, 0
 ; GFX11_W64-NEXT:    s_and_not1_b64 s[6:7], 0, exec
-; GFX11_W64-NEXT:    s_and_b64 s[0:1], exec, s[0:1]
 ; GFX11_W64-NEXT:    s_or_b64 vcc, s[6:7], s[0:1]
 ; GFX11_W64-NEXT:  .LBB13_2: ; %exit
 ; GFX11_W64-NEXT:    s_or_b64 exec, exec, s[2:3]

diff --git a/llvm/test/CodeGen/AMDGPU/fold-and-exec.mir b/llvm/test/CodeGen/AMDGPU/fold-and-exec.mir
@@ -258,3 +258,24 @@ body: |
     S_ENDPGM 0, implicit $vcc
 ...
 
+# S_CSELECT EXEC, 0 is itself masked by EXEC, so ANDing it with EXEC is a no-op.
+---
+name: cselect_exec_zero
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $sgpr0
+    ; CHECK-LABEL: name: cselect_exec_zero
+    ; CHECK: liveins: $sgpr0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: S_CMP_LG_U32 $sgpr0, 0, implicit-def $scc
+    ; CHECK-NEXT: [[S_CSELECT_B64_:%[0-9]+]]:sreg_64_xexec = S_CSELECT_B64 $exec, 0, implicit $scc
+    ; CHECK-NEXT: $vcc = COPY [[S_CSELECT_B64_]]
+    ; CHECK-NEXT: S_ENDPGM 0, implicit $vcc
+    S_CMP_LG_U32 $sgpr0, 0, implicit-def $scc
+    %0:sreg_64_xexec = S_CSELECT_B64 $exec, 0, implicit $scc
+    %1:sreg_64_xexec = S_AND_B64 $exec, %0, implicit-def dead $scc
+    $vcc = COPY %1
+    S_ENDPGM 0, implicit $vcc
+...
+