[RISCV] Guard against out of bound shifts in expandMul.

[topperc]

Spotted while reviewing #150211. If we're multiplying by -3 in i32 MulAmt contains 4,294,967,293 since we zero extend to uint64_t. Adding 3 to this gives 0x100000000 which is a power of 2 and the log2 of that is 32, but we can't shift left by 32 in an i32.

Detect this case and skip the transform. We could use 0, but we don't handle the case for i64 so this seemed more consistent.

Normally we don't hit this case because decomposeMulByConstant handles it, but that's disabled by Xqciac. And after #150211 the path in expandMul will also be unreachable. So I didn't add a test to avoid messing with that review.

[llvmbot]

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

Author: Craig Topper (topperc)

Changes

Spotted while reviewing #150211. If we're multiplying by -3 in i32 MulAmt contains 4,294,967,293 since we zero extend to uint64_t. Adding 3 to this gives 0x100000000 which is a power of 2 and the log2 of that is 32, but we can't shift left by 32 in an i32.

We should use 0 instead of the shl in this case.

Normally we don't hit this case because decomeMulByConstant handles it, but that's disabled by Qciac. And after #150211 the path in expandMul will also be unreachable. So I didn't add a test to avoid messing with that review.

---

Full diff: https://github.com/llvm/llvm-project/pull/150464.diff

1 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+7-2)

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 3918dd21bc09d..25cb230785f72 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -16184,10 +16184,15 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
     // 2^N - 3/5/9 --> (sub (shl X, C1), (shXadd X, x))
     for (uint64_t Offset : {3, 5, 9}) {
       if (isPowerOf2_64(MulAmt + Offset)) {
+        unsigned ShAmt = Log2_64(MulAmt + Offset);
         SDLoc DL(N);
-        SDValue Shift1 =
+        SDValue Shift1;
+        if (ShAmt >= VT.getSizeInBits())
+          Shift1 = DAG.getConstant(0, DL, VT);
+        else
+          Shift1 =
             DAG.getNode(ISD::SHL, DL, VT, X,
-                        DAG.getConstant(Log2_64(MulAmt + Offset), DL, VT));
+                        DAG.getConstant(ShAmt, DL, VT));
         SDValue Mul359 =
             DAG.getNode(RISCVISD::SHL_ADD, DL, VT, X,
                         DAG.getConstant(Log2_64(Offset - 1), DL, VT), X);

[svs-quic]

LGTM! Thanks for fixing this.