<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=http://email.email.llvm.org/c/eJy1Vttu4zgM_ZrkhXDg2Lk-5CFN2t0AnbaYdgfYp0C2aVs7jmRIctvM1--RnHbSueyt2ECIJYoiKZ4jSpkujqvdIJkfqJbOSVWRoNwIW9NTzYpyfWhl48VP0tVUcNZVJFWpSStyNdNBSEWZESqvR4N4O4jXg1l8amGYN0JVg3RNTfN4iFqj_-DcDZIrP_QfmeF_owv-hRV6a3u4M1I5Nhhsn4QpL59bw9ZKrUZ528LSNF54e5nWTTCKAdo3ur1QFMWZKFmcqe--Tmw6Y7WhQTILbXNmtexU7qCyN1wO0k3wCTudsrJSXCAVzi_4mbXlIL0M_2taW8vGmyKk5l5-4Z26kM6S10i3dFuWlt1JFtGVEdWBlevFp9DwSSxWUqcKNmWjnyAAdlQK2XDx4_z3_78BMuM6JRw3Rx_xzuMNrJ9E0_RQO3P0ODtNVh7aRpbHALBhYFZ0OZsR7RxZ5oP1SgW3rApPA0GPbI7UCFMx-Wh1GXjTyBzuCrIOH78ZCwuUCwAxd9R2WSNBM-8jB_xUs2HKOBedZUJs0hI8t0ayEybEnHWu18JUWOapBQgeNRz5zPZE7iz28RdkpAjt9gtFkT1ao7VD_kE23XpaClUYLYu9Kj5jZMbzMkOKr06aFDm_SyROmLyeTSKlFUc4H91zdFqZjCna7q93F5u7u_2v6_v9ze3-0-XN9vbjfv1pvbteX-yudw-_79c3N7cP64fd7c09RSV2WgLDqGLFBumiKMd2q8D4n6O6Q267QiLHr1kU1OAcN-gAnFKDKRC1wuU-0wJpN_zooQgr-sOMLhKaC8svSd5RoXuUAtMAIMwAEla6q2qPfi0evUNVyoJVzr3tHrRAGlnV4IpuOo_LiD4cUT-agqoOZwScs_2C4NiPBHnMmpBKjmQZvXImxOjdhwXY_0eupPXlIZzP-y6DwB8nTHnvYINoJKItv1cOJQKiG31S_sJGf0uUQpZlL_I9kKTy5-S_FyzK3lHsQhwSGDxTOp_E83y54Pm0HI14VvBUTNKMszmN43g2mfTKURS9J9pTMpKLvr0_-MEkRqMoWSzAriVK2EXfHcd0mgtF9Wfl-4PIa3ACqL0p5KCedfQQDuMLyDt_LaFMPnxEHdj07okCE5JFaGNp74QRB4b2J9F0HKTLr-UVCh_4oM3x-lRSvlP4VdiNr238fB4PNOYb316c-p-0lwpF9cxRr3hxplWGm2IhOhdCx1bIXxYvTLVvVkQnu2FDmB19JfTLNRJ7Ly8Y_jP1byLyP-Qdat-t8uXBS5AAfzTDxfY2NCy8be8k53xafHY-z7RfY_v3Kwbz7Vmwr6MzWb_fv0n-j8tp_w4SbXv0Nc4xKKZRDoU6kkZZM1TKZ8Yt9lAL9RnYjGmz8SwWhZFC4XEjlGtoWKzSYpkuxRCw1tqsuPE3XTXsTLOqnWutZ3dyhYbaUnfZCLfl2Tl7-06S1qJoojNN00kyrFdpucyQtTQTSbmMC8HJJBVptpiJvBwvcx42IuPGrgZTnOBE8RMFE_6pMN0O3x-BXCVxksTjZDxeTNMkHZWzosyWKEhjFmKGXE9ixrOwGXk7I22qoVkFkyjlFpNIBp4Cr5PChrcUh4ARoZO4vlY_fvr9f68oXAinV9Qw7HcVNvsnZV9-_w>53342</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            DwarfExpression.cpp: Assertion `SizeInBits >= OffsetInBits - FragmentOffset && "size underflow"` failed.
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ellishg
      </td>
    </tr>
</table>

<pre>
    I'm hitting a crash when compiling with debug info on the main branch.
```
clang: llvm-project/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp:508: bool llvm::DwarfExpression::addExpression(llvm::DIExpressionCursor &&, llvm::function_ref<bool (unsigned int, DIExpressionCursor &)>): Assertion `SizeInBits >= OffsetInBits - FragmentOffset && "size underflow"' failed.
```

Unfortunately, I hit a wall when trying to simplify the reproducer. It seems to depend on a very large set of complicated statements. I can't publish the code here because it is proprietary, but here is the clang invocation I'm using.
```
clang -g -Oz --sysroot=/opt/android_ndk/r17fb2/sysroot -target aarch64-none-linux-android21 -D_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS -fprofile-generate -c bug.cpp
```

I studied the code a little and found a patch that prevents the bug in this case, but I don't understand it enough to have confidence that it is the right solution. My wild guess was that this was a single-line-if-statement bug and that `Register::SubRegSize` is invalid if `Register::DwarfRegNo` is zero.
```diff
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
index 37407c98e75f..e6de5a43beb7 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -288,9 +288,10 @@ bool DwarfExpression::addMachineRegExpression(const TargetRegisterInfo &TRI,
   if ((!isParameterValue() && !isMemoryLocation() && !HasComplexExpression) ||
       isEntryValue()) {
     for (auto &Reg : DwarfRegs) {
-      if (Reg.DwarfRegNo >= 0)
+      if (Reg.DwarfRegNo >= 0) {
         addReg(Reg.DwarfRegNo, Reg.Comment);
-      addOpPiece(Reg.SubRegSize);
+        addOpPiece(Reg.SubRegSize);
+      }
     }

     if (isEntryValue()) {
```

I'm happy to test out any other fixes. Thanks! CC @adrian-prantl 
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy1Vttu2zgQ_Rr7ZSDDlu8PfnDsZGsgTYImLbBPBiWOJG5lUiCpJO7X7yHlpE4veyvWICxehjPDOTOHzIw8rna9dH6gSnmvdEmCcitcRU8Va8rNoVF1mH5SviLJWVuS0oUho8lXTAehNGVW6Lwa9Ibb3nDdmw1PLQ7zWuiyN15TXT8eksaaPzj3vfQqDMNHZfjfGMm_sUZv7Q53VmnPFoPtk7DF5XNj2Tll9CBvGmiaDhdBX2ZMHZVigPaNbDcppDybShdn4ruvC5vWOmOpl85i25xpLVqde4jsLRe98SbahJ5WO1VqlgiFDxt-pm3ZG1_G_zWtnWMbVBFCc6--8E5fKO8oSIy3dFsUjv1pLqErK8oDa99Nn1zDJ3XYSa2WbIvaPGEC2FEhVM3yx_Hv_j8CMutbLTzXx-DxLuANrJ9EXXdQe3sMOHtDTh2aWhXHCLBlYCbbnO2Adp4c88EFIckNaxnSQNAj2yPVwpZMwVtTxLypVQ5zkpzHJxzGQQPlAkDMPTVtViukWbCRA36q2DJlnIvWMcE35QiWG6vYCxt9zlrfSWEpbgupBQgeDQyFyHaJ3Dqc4y-SkRK02y-UJO7orDEe8UeymSakpdDSGiX3Wn7GyI7mRYYQX50kKfHhlAicsHk1myTaaE5QH-1zctqZjijZ7q93F5u7u_279f3-5nb_6fJme_thv_603l2vL3bXu4ff9-ubm9uH9cPu9uaekgInLYBhUrJmi3BRkuO4Zcz4n6O6Q2xbqRDj1ygKqlHHNToApzDIFEw1wuch0gJht_wYoIg7umJGFwHNheOXIO9Img6lmGkAEGoACWvTllVAvxKPwaAulGSdc6e7Ay0mjSor5Iqp24DLgN4fwR-1pLJFjSDnXLchGg4jQQGzOoaSE1UkrzkTfQzm4wac_wOXygV6iPV532aYCOWEpWAd2SBqBW-L74UjRWDqxpyEv7A13yaKVEXRTYUekqQMdfLfCYuyXyC76IcCBs80nk-G83y54Pm0GAx4JnkqJuOMszmNhsPZZNIJJ0nyK96egpFedO3Xne9NhmiUpIsFsmsJCrvouqMhndYiqf6Mvt-LvEJOALU3RI7Uc54eYjG-gLwL1xJo8uEDeGDTmSeKmZAuYhspdyesODCkP4m65Ti7_EqvEHjPB2OP1ydK-U7gnXCbwG38fO4PJOab0F6Mhp9ylxqkemaoE7w4kyriTbEQrY-u4ygULouXTHVvdiQnvfFAWB18TeiXa2QYrLxg-M_Ev_Eo_BB3iH23K9BDmEEAQmnGi-2ta9h429wpzvm0-aw-z6Rfffv3O3rz7Zmzr6Ozue68fxP8H9Np9w4STXMMHOcZKWZAh0IfyYDWLBXqmXGLPVRCfwY2I9psQhYLaZXQeNwI7Wvqy9VYLsdL0fcKTLz68Svm_3sQgNtOD4J-a-tV5X3jQj2lV2hgs6rNBrifzyr77ctMOQeaRmc6Hk_SfrWaz5eLQvBULgqQjpjOxVTKSZou5qN8LEazfi0yrt2qNwVnpJqfKKoIvky3fbVKh2k6HKWj0WI6TseDYiaLbAkCG7EQM2AzGTKekfUg-DEwtuzbVXQJ1O-wiGcCng6vi8LFtxdHc9CP2qmMXXEdnhNlP5peRdf_BHjRaLY">