[RISCV] Teach computeKnownBits that vsetvli returns <= 65536.

Resolves a FIXME. We could do even better taking into account SEW/LMUL.

Author: topperc

llvm/lib/Analysis/ValueTracking.cpp

@@ -1738,10 +1738,10 @@ static void computeKnownBitsFromOperator(const Operator *I,
       case Intrinsic::riscv_vsetvli_opt:
       case Intrinsic::riscv_vsetvlimax:
       case Intrinsic::riscv_vsetvlimax_opt:
-        // Assume that VL output is positive and would fit in an int32_t.
-        // TODO: VLEN might be capped at 16 bits in a future V spec update.
-        if (BitWidth >= 32)
-          Known.Zero.setBitsFrom(31);
+        // Assume that VL output is >= 65536.
+        // TODO: Take SEW and LMUL into account.
+        if (BitWidth > 17)
+          Known.Zero.setBitsFrom(17);
         break;
       case Intrinsic::vscale: {
         if (!II->getParent() || !II->getFunction() ||

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -10887,10 +10887,10 @@ void RISCVTargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     case Intrinsic::riscv_vsetvlimax:
     case Intrinsic::riscv_vsetvli_opt:
     case Intrinsic::riscv_vsetvlimax_opt:
-      // Assume that VL output is positive and would fit in an int32_t.
-      // TODO: VLEN might be capped at 16 bits in a future V spec update.
-      if (BitWidth >= 32)
-        Known.Zero.setBitsFrom(31);
+      // Assume that VL output is >= 65536.
+      // TODO: Take SEW and LMUL into account.
+      if (BitWidth > 17)
+        Known.Zero.setBitsFrom(17);
       break;
     }
     break;

llvm/test/CodeGen/RISCV/rvv/vsetvl-ext.ll

@@ -23,3 +23,13 @@ define zeroext i32 @vsetvl_zext() {
   %b = trunc i64 %a to i32
   ret i32 %b
 }
+
+define i64 @vsetvl_and17bits() {
+; CHECK-LABEL: vsetvl_and17bits:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli a0, 1, e16, m2, ta, mu
+; CHECK-NEXT:    ret
+  %a = call i64 @llvm.riscv.vsetvli(i64 1, i64 1, i64 1)
+  %b = and i64 %a, 131071
+  ret i64 %b
+}

llvm/test/Transforms/InstCombine/RISCV/riscv-vsetvli-knownbits.ll

@@ -48,6 +48,30 @@ entry:
   ret i64 %2
 }
 
+define i32 @vsetvli_and17_i32() nounwind {
+; CHECK-LABEL: @vsetvli_and17_i32(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.riscv.vsetvli.i32(i32 1, i32 1, i32 1)
+; CHECK-NEXT:    ret i32 [[TMP0]]
+;
+entry:
+  %0 = call i32 @llvm.riscv.vsetvli.i32(i32 1, i32 1, i32 1)
+  %1 = and i32 %0, 131071
+  ret i32 %1
+}
+
+define i64 @vsetvli_and17_i64() nounwind {
+; CHECK-LABEL: @vsetvli_and17_i64(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.riscv.vsetvli.i64(i64 1, i64 1, i64 1)
+; CHECK-NEXT:    ret i64 [[TMP0]]
+;
+entry:
+  %0 = call i64 @llvm.riscv.vsetvli.i64(i64 1, i64 1, i64 1)
+  %1 = and i64 %0, 131071
+  ret i64 %1
+}
+
 define i32 @vsetvlimax_i32() nounwind {
 ; CHECK-LABEL: @vsetvlimax_i32(
 ; CHECK-NEXT:  entry:
@@ -86,6 +110,30 @@ entry:
   ret i64 %2
 }
 
+define i32 @vsetvlimax_and17_i32() nounwind {
+; CHECK-LABEL: @vsetvlimax_and17_i32(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.riscv.vsetvlimax.i32(i32 1, i32 1)
+; CHECK-NEXT:    ret i32 [[TMP0]]
+;
+entry:
+  %0 = call i32 @llvm.riscv.vsetvlimax.i32(i32 1, i32 1)
+  %1 = and i32 %0, 131071
+  ret i32 %1
+}
+
+define i64 @vsetvlimax_and17_i64() nounwind {
+; CHECK-LABEL: @vsetvlimax_and17_i64(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.riscv.vsetvlimax.i64(i64 1, i64 1)
+; CHECK-NEXT:    ret i64 [[TMP0]]
+;
+entry:
+  %0 = call i64 @llvm.riscv.vsetvlimax.i64(i64 1, i64 1)
+  %1 = and i64 %0, 131071
+  ret i64 %1
+}
+
 define i32 @vsetvli_opt_i32() nounwind {
 ; CHECK-LABEL: @vsetvli_opt_i32(
 ; CHECK-NEXT:  entry:
@@ -124,6 +172,30 @@ entry:
   ret i64 %2
 }
 
+define i32 @vsetvli_opt_and17_i32() nounwind {
+; CHECK-LABEL: @vsetvli_opt_and17_i32(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.riscv.vsetvli.opt.i32(i32 1, i32 1, i32 1)
+; CHECK-NEXT:    ret i32 [[TMP0]]
+;
+entry:
+  %0 = call i32 @llvm.riscv.vsetvli.opt.i32(i32 1, i32 1, i32 1)
+  %1 = and i32 %0, 131071
+  ret i32 %1
+}
+
+define i64 @vsetvli_opt_and17_i64() nounwind {
+; CHECK-LABEL: @vsetvli_opt_and17_i64(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.riscv.vsetvli.opt.i64(i64 1, i64 1, i64 1)
+; CHECK-NEXT:    ret i64 [[TMP0]]
+;
+entry:
+  %0 = call i64 @llvm.riscv.vsetvli.opt.i64(i64 1, i64 1, i64 1)
+  %1 = and i64 %0, 131071
+  ret i64 %1
+}
+
 define i32 @vsetvlimax_opt_i32() nounwind {
 ; CHECK-LABEL: @vsetvlimax_opt_i32(
 ; CHECK-NEXT:  entry:
@@ -161,3 +233,27 @@ entry:
   %2 = zext i32 %1 to i64
   ret i64 %2
 }
+
+define i32 @vsetvlimax_opt_and17_i32() nounwind {
+; CHECK-LABEL: @vsetvlimax_opt_and17_i32(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.riscv.vsetvlimax.opt.i32(i32 1, i32 1)
+; CHECK-NEXT:    ret i32 [[TMP0]]
+;
+entry:
+  %0 = call i32 @llvm.riscv.vsetvlimax.opt.i32(i32 1, i32 1)
+  %1 = and i32 %0, 131071
+  ret i32 %1
+}
+
+define i64 @vsetvlimax_opt_and17_i64() nounwind {
+; CHECK-LABEL: @vsetvlimax_opt_and17_i64(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.riscv.vsetvlimax.opt.i64(i64 1, i64 1)
+; CHECK-NEXT:    ret i64 [[TMP0]]
+;
+entry:
+  %0 = call i64 @llvm.riscv.vsetvlimax.opt.i64(i64 1, i64 1)
+  %1 = and i64 %0, 131071
+  ret i64 %1
+}