RyuJIT x86: GT_STORE_LCL_VAR decomposition can generate wrong IR

[mikedn]

Running the following example results in an assert:

using System;
using System.Runtime.CompilerServices;

class Program
{
    uint i;

    [MethodImpl(MethodImplOptions.NoInlining)]
    ulong Test(uint h)
    {
        uint x = h + 4;
        ulong f = checked((ulong)unchecked(x - i));
        if (i > h)
            return 0;
        return f;
    }

    static void Main()
    {
        new Program().Test(42);
    }
}

Assert failure(PID 9708 [0x000025ec], Thread: 9964 [0x26ec]): Assertion failed 'treeNodeCount == linearNodeCount' in 'Program:Test(int):long:this' (IL size 26)

    File: d:\projects\coreclr\src\jit\flowgraph.cpp Line: 20861
    Image: D:\Projects\coreclr\bin\Product\Windows_NT.x86.Debug\CoreRun.exe

The IR contains an embedded statement and GT_STORE_LCL_VAR decomposition attempts to insert a statement after the current one using fgInsertStmtAfter. But that results in the new statement being inserted between the current statement and its embedded statement.

Relevant IR:

***** BB01, stmt 1 (top level)
     ( 23, 21) [000010] ------------             *  stmtExpr  void  (top level) (IL 0x000...0x00B)
N001 (  1,  1) [000001] ------------             |           /--*  lclVar    int    V01 arg1         u:2 $c0
N002 (  1,  1) [000002] ------------             |           +--*  const     int    4 $43
N003 (  3,  3) [000003] ------------             |        /--*  +         int    $140
     (  8,  7) [000037] ------------             |        |  {  *  stmtExpr  void  (embedded) (IL 0x000...  ???)
N004 (  1,  1) [000004] ------------             |        |  {  |        /--*  lclVar    ref    V00 this         u:2 $80
N005 (  1,  1) [000026] ------------             |        |  {  |        +--*  const     int    4 field offset Fseq[i] $43
N006 (  2,  2) [000027] -------N----             |        |  {  |     /--*  +         byref  $180
N007 (  4,  4) [000005] ---XG-------             |        |  {  |  /--*  indir     int    <l:$142, c:$200>
N009 (  8,  7) [000033] DA-XG-------             |        |  {  \--*  st.lclVar int    V03 cse0
N010 (  3,  2) [000034] ------------             |        +--*  lclVar    int    V03 cse0          <l:$142, c:$200>
N012 ( 15, 13) [000006] -A-XG-------             |     /--*  -         int    <l:$144, c:$143>
N013 ( 16, 15) [000007] -A-XG----U--             |  /--*  cast      long <- ulong <- uint <l:$241, c:$240>
N015 ( 23, 21) [000009] DA-XG-------             \--*  st.lclVar long   V02 loc0         d:3

[CarolEidt]

@mikedn - Thanks for the quick response!