[llvm] [RISCV][VLOPT] Set CommonVL as the largest of the users (PR #120349)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Michael Maitland (michaelmaitland)

<details>
<summary>Changes</summary>

Prior to this patch, we required that all users had the same VL in order to optimize. But as the FIXME said, we can use the largest VL to optimize, as long as we can determine what the largest is. This patch implements the FIXME.

---
Full diff: https://github.com/llvm/llvm-project/pull/120349.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp (+5-5) 
- (modified) llvm/test/CodeGen/RISCV/rvv/vl-opt.mir (+78) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
index e8719d02cfa0aa..f4182eda81fdae 100644
--- a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
+++ b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -936,13 +936,13 @@ bool RISCVVLOptimizer::checkUsers(const MachineOperand *&CommonVL,
     assert((!VLOp.isReg() || VLOp.getReg() != RISCV::X0) &&
            "Did not expect X0 VL");
 
-    if (!CommonVL) {
+    // Use the largest VL among all the users. If we cannot determine this
+    // statically, then we cannot optimize the VL.
+    if (!CommonVL || RISCV::isVLKnownLE(*CommonVL, VLOp)) {
       CommonVL = &VLOp;
       LLVM_DEBUG(dbgs() << "    User VL is: " << VLOp << "\n");
-    } else if (!CommonVL->isIdenticalTo(VLOp)) {
-      // FIXME: This check requires all users to have the same VL. We can relax
-      // this and get the largest VL amongst all users.
-      LLVM_DEBUG(dbgs() << "    Abort because users have different VL\n");
+    } else if (!RISCV::isVLKnownLE(VLOp, *CommonVL)) {
+      LLVM_DEBUG(dbgs() << "    Abort because cannot determine a common VL\n");
       CanReduceVL = false;
       break;
     }
diff --git a/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir b/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
index 010e3ca642269b..3f966b036589fd 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
+++ b/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
@@ -33,3 +33,81 @@ body: |
     %y:vr = PseudoVREDSUM_VS_M1_E64 $noreg, %x, $noreg, -1, 6 /* e64 */, 0 /* tu, mu */
     %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 5 /* e32 */, 0 /* tu, mu */
 ...
+---
+name: use_largest_common_vl_imm_imm
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: use_largest_common_vl_imm_imm
+    ; CHECK: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 2, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 2, 3 /* e8 */, 0 /* tu, mu */
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0
+    %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 1, 3 /* e8 */, 0
+    %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 2, 3 /* e8 */, 0
+...
+---
+name: use_largest_common_vl_same_reg
+body: |
+  bb.0:
+    liveins: $x1
+    ; CHECK-LABEL: name: use_largest_common_vl_same_reg
+    ; CHECK: liveins: $x1
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %vl:gprnox0 = COPY $x1
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, %vl, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 3 /* e8 */, 0 /* tu, mu */
+    %vl:gprnox0 = COPY $x1
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0
+    %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 3 /* e8 */, 0
+    %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 3 /* e8 */, 0
+...
+---
+name: use_largest_common_vl_diff_regs
+body: |
+  bb.0:
+    liveins: $x1, $x2
+    ; CHECK-LABEL: name: use_largest_common_vl_diff_regs
+    ; CHECK: liveins: $x1, $x2
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %vl0:gprnox0 = COPY $x1
+    ; CHECK-NEXT: %vl1:gprnox0 = COPY $x2
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl0, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl1, 3 /* e8 */, 0 /* tu, mu */
+    %vl0:gprnox0 = COPY $x1
+    %vl1:gprnox0 = COPY $x2
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0
+    %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl0, 3 /* e8 */, 0
+    %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl1, 3 /* e8 */, 0
+...
+---
+name: use_largest_common_vl_imm_reg
+body: |
+  bb.0:
+    liveins: $x1
+    ; CHECK-LABEL: name: use_largest_common_vl_imm_reg
+    ; CHECK: liveins: $x1
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %vl:gprnox0 = COPY $x1
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+    %vl:gprnox0 = COPY $x1
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0
+    %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, %vl, 3 /* e8 */, 0
+    %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 1, 3 /* e8 */, 0
+...
+---
+name: use_largest_common_vl_imm_vlmax
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: use_largest_common_vl_imm_vlmax
+    ; CHECK: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0
+    %y:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, 1, 3 /* e8 */, 0
+    %z:vr = PseudoVADD_VV_M1 $noreg, %x, $noreg, -1, 3 /* e8 */, 0
+...
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/120349