[llvm] [AMDGPU][True16][CodeGen] readfirstlane for vgpr16 copy to sgpr32 (PR #137848)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Brox Chen (broxigarchen)

<details>
<summary>Changes</summary>

i16 can be selected into sgpr32 or vgpr16 in isel lowering in true16 mode. And thus, it creates cases that we copy from vgpr16 to sgpr32 in ext selection.

Add legalization when we decide to lower this copy to readfirstlane in fix-sgpr-copy pass and add a lit test

---
Full diff: https://github.com/llvm/llvm-project/pull/137848.diff


3 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp (+17-4) 
- (modified) llvm/lib/Target/AMDGPU/VOP1Instructions.td (+1-8) 
- (added) llvm/test/CodeGen/AMDGPU/fix-sgpr-copies-vgpr16-to-spgr32.ll (+41) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp b/llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp
index bb8e9a092e07c..47052a80bf125 100644
--- a/llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp
@@ -1086,10 +1086,23 @@ void SIFixSGPRCopies::lowerVGPR2SGPRCopies(MachineFunction &MF) {
         TRI->getRegClassForOperandReg(*MRI, MI->getOperand(1));
     size_t SrcSize = TRI->getRegSizeInBits(*SrcRC);
     if (SrcSize == 16) {
-      // HACK to handle possible 16bit VGPR source
-      auto MIB = BuildMI(*MBB, MI, MI->getDebugLoc(),
-                         TII->get(AMDGPU::V_READFIRSTLANE_B32), DstReg);
-      MIB.addReg(SrcReg, 0, AMDGPU::NoSubRegister);
+      assert(MF.getSubtarget<GCNSubtarget>().useRealTrue16Insts() &&
+             "We do not expect to see 16-bit copies from VGPR to SGPR unless "
+             "we have 16-bit VGPRs");
+      assert(MRI->getRegClass(DstReg) == &AMDGPU::SGPR_LO16RegClass ||
+             MRI->getRegClass(DstReg) == &AMDGPU::SReg_32RegClass ||
+             MRI->getRegClass(DstReg) == &AMDGPU::SReg_32_XM0RegClass);
+      // There is no V_READFIRSTLANE_B16, so legalize the dst/src reg to 32 bits
+      if (MRI->getRegClass(DstReg) == &AMDGPU::SGPR_LO16RegClass)
+        MRI->setRegClass(DstReg, &AMDGPU::SReg_32RegClass);
+      Register VReg32 = MRI->createVirtualRegister(&AMDGPU::VGPR_32RegClass);
+      const DebugLoc &DL = MI->getDebugLoc();
+      BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::SUBREG_TO_REG), VReg32)
+          .addImm(0)
+          .addReg(SrcReg, 0)
+          .addImm(AMDGPU::lo16);
+      BuildMI(*MBB, MI, DL, TII->get(AMDGPU::V_READFIRSTLANE_B32), DstReg)
+          .addReg(VReg32);
     } else if (SrcSize == 32) {
       auto MIB = BuildMI(*MBB, MI, MI->getDebugLoc(),
                          TII->get(AMDGPU::V_READFIRSTLANE_B32), DstReg);
diff --git a/llvm/lib/Target/AMDGPU/VOP1Instructions.td b/llvm/lib/Target/AMDGPU/VOP1Instructions.td
index 071f55ce16403..352a3f9c2d27f 100644
--- a/llvm/lib/Target/AMDGPU/VOP1Instructions.td
+++ b/llvm/lib/Target/AMDGPU/VOP1Instructions.td
@@ -1472,16 +1472,9 @@ def : GCNPat <
 
 } // End OtherPredicates = [isGFX8Plus, p]
 
-let True16Predicate = UseFakeTrue16Insts in {
-def : GCNPat<
-  (i32 (DivergentUnaryFrag<anyext> i16:$src)),
-  (COPY $src)
->;
-} // End True16Predicate = UseFakeTrue16Insts
-
 let True16Predicate = UseRealTrue16Insts in {
 def : GCNPat<
-  (i32 (UniformUnaryFrag<anyext> (i16 SReg_32:$src))),
+  (i32 (UniformUnaryFrag<anyext> i16:$src)),
   (COPY $src)
 >;
 
diff --git a/llvm/test/CodeGen/AMDGPU/fix-sgpr-copies-vgpr16-to-spgr32.ll b/llvm/test/CodeGen/AMDGPU/fix-sgpr-copies-vgpr16-to-spgr32.ll
new file mode 100644
index 0000000000000..0b42274f9553d
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/fix-sgpr-copies-vgpr16-to-spgr32.ll
@@ -0,0 +1,41 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 3
+; RUN: llc -mtriple=amdgcn -mcpu=gfx1100 -mattr=+real-true16 < %s | FileCheck %s
+
+; expect readfirstlane to pick the 32bit register
+define amdgpu_gs i32 @vgpr16_copyto_sgpr(ptr addrspace(3) %a, i32 %b, ptr addrspace(1) %out) {
+; CHECK-LABEL: vgpr16_copyto_sgpr:
+; CHECK:       ; %bb.0: ; %entry
+; CHECK-NEXT:    ds_load_2addr_b32 v[0:1], v0 offset1:1
+; CHECK-NEXT:    s_waitcnt lgkmcnt(0)
+; CHECK-NEXT:    v_cvt_f16_f32_e32 v0.l, v0
+; CHECK-NEXT:    s_delay_alu instid0(VALU_DEP_1) | instskip(SKIP_1) | instid1(SALU_CYCLE_1)
+; CHECK-NEXT:    v_readfirstlane_b32 s0, v0
+; CHECK-NEXT:    s_and_b32 s0, 0xffff, s0
+; CHECK-NEXT:    s_mul_i32 s0, s0, 5
+; CHECK-NEXT:    s_delay_alu instid0(SALU_CYCLE_1)
+; CHECK-NEXT:    s_cmp_lg_u32 s0, 2
+; CHECK-NEXT:    s_cbranch_scc1 .LBB0_2
+; CHECK-NEXT:  ; %bb.1: ; %a1
+; CHECK-NEXT:    s_mov_b32 s0, 1
+; CHECK-NEXT:    s_branch .LBB0_3
+; CHECK-NEXT:  .LBB0_2: ; %a2
+; CHECK-NEXT:    s_mov_b32 s0, 2
+; CHECK-NEXT:    s_branch .LBB0_3
+; CHECK-NEXT:  .LBB0_3:
+entry:
+  %1 = load <4 x float>, ptr addrspace(3) poison, align 4
+  %2 = extractelement <4 x float> %1, i32 0
+  %3 = fptrunc float %2 to half
+  %4 = bitcast half %3 to i16
+  %5 = zext i16 %4 to i32
+  %6 = add i32 %5, 1
+  %7 = mul i32 %6, 5
+  %8 = icmp eq i32 %7, 7
+  br i1 %8, label %a1, label %a2
+
+a1:
+  ret i32 1
+
+a2:
+  ret i32 2
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/137848