Foo.cs

// compiled with:
//  csc /target:library /unsafe /O foo.cs


using System;
using System.Runtime.InteropServices;


public struct S {
	public int a;
	public int b;
}

public unsafe class P {
	[DllImport ("Bar")]
	static extern void Bar (int *x, int *y);

	public static void Plop (S[] x, bool b)
	{
		if (b) {
			fixed (S* p = x)
			{
				Bar (&p->a, &p->b);
			}
			Plop (x, !b);
		} else
			Plop (x, !b);
				
	}
}

monodis-output.il

   // method line 2
    .method public static hidebysig 
           default void Plop (valuetype S[] x, bool b)  cil managed 
    {
        // Method begins at RVA 0x2050
	// Code size 70 (0x46)
	.maxstack 3
	.locals init (
		valuetype S*	V_0,
		valuetype S[] pinned	V_1)
	IL_0000:  ldarg.1 
	IL_0001:  brfalse.s IL_003b

	IL_0003:  ldarg.0 
	IL_0004:  dup 
	IL_0005:  stloc.1 
	IL_0006:  brfalse.s IL_000d

	IL_0008:  ldloc.1 
	IL_0009:  ldlen 
	IL_000a:  conv.i4 
	IL_000b:  brtrue.s IL_0012

	IL_000d:  ldc.i4.0 
	IL_000e:  conv.u 
	IL_000f:  stloc.0 
	IL_0010:  br.s IL_001b

	IL_0012:  ldloc.1 
	IL_0013:  ldc.i4.0 
	IL_0014:  ldelema S
	IL_0019:  conv.u 
	IL_001a:  stloc.0 
	IL_001b:  ldloc.0 
	IL_001c:  ldflda int32 S::a
	IL_0021:  conv.u 
	IL_0022:  ldloc.0 
	IL_0023:  ldflda int32 S::b
	IL_0028:  conv.u 
	IL_0029:  call void class P::Bar(int32*, int32*)
	IL_002e:  ldnull 
	IL_002f:  stloc.1                                   // <- zeroes out the fixed pointer
	IL_0030:  ldarg.0 
	IL_0031:  ldarg.1 
	IL_0032:  ldc.i4.0 
	IL_0033:  ceq 
	IL_0035:  call void class P::Plop(valuetype S[], bool)
	IL_003a:  ret 
	IL_003b:  ldarg.0 
	IL_003c:  ldarg.1 
	IL_003d:  ldc.i4.0 
	IL_003e:  ceq 
	IL_0040:  call void class P::Plop(valuetype S[], bool)
	IL_0045:  ret 
    } // end of method P::Plop

verbose-method-liveness-iteration.txt

Var R16 live at enter, add range to R16: [1c0001, 200001)
Var R17 live at enter, add range to R17: [1c0001, 200001)
Var R19 live at enter, add range to R19: [1c0001, 200001)
LIVENESS BLOCK BB9:
	180013:  br [B1]
	180012:  il_seq_point il: 0x3a
	180011:  il_seq_point il: 0x3a, nonempty-stack
	180010:  voidcall [void P:Plop (S[],bool)] [%rdi <- R49] [%rsi <- R50] clobbers: c
	18000f:  move R50 <- R48
	18000e:  move R49 <- R16
	last use of R16 set to 18000e
	18000d:  int_ceq R48 <-
	18000c:  icompare_imm R17 [0]
	last use of R17 set to 18000c
	18000b:  il_seq_point il: 0x30
	18000a:  i8const R19 <- [0]
	dead def of R19, eliminated                                              // store to fixed pointer eliminated!
	180009:  il_seq_point il: 0x2e
	180008:  il_seq_point il: 0x2e, nonempty-stack
	180007:  voidcall [void P:Bar (int*,int*)] [%rdi <- R42] [%rsi <- R43] clobbers: c
	180006:  move R43 <- R41
	180005:  move R42 <- R18
	last use of R18 set to 180005
	180004:  long_add_imm R41 <- R18 [4] clobbers: 1
	180003:  il_seq_point il: 0x1b

verbose-method-liveness.txt

                                        // I believe R19 is the fixed pointer
LIVENESS:
BLOCK BB0 (BB3, ):
	1  aotconst R57 <-
	1  gc_safe_point R57 clobbers: c
GEN  BB0: {}
KILL BB0: {}
BLOCK BB3 (BB4, BB5, ):
	1  i8const R18 <- [0]
	KILL: R18(2)
	1  i8const R19 <- [0]
	KILL: R19(3)
	1  il_seq_point il: 0x0
	1  icompare_imm R17 [0]
	GEN: R17(1)
	1  int_beq [B4B5]
GEN  BB3: {1}
KILL BB3: {2, 3}
BLOCK BB4 (BB1, ):
	1  il_seq_point il: 0x3b
	1  icompare_imm R17 [0]
	GEN: R17(1)
	1  int_ceq R54 <-
	1  move R55 <- R16
	GEN: R16(0)
	1  move R56 <- R54
	1  voidcall [void P:Plop (S[],bool)] [%rdi <- R55] [%rsi <- R56] clobbers: c
	1  il_seq_point il: 0x45, nonempty-stack
	1  il_seq_point il: 0x45
GEN  BB4: {0, 1}
KILL BB4: {}
BLOCK BB1 ():
GEN  BB1: {}
KILL BB1: {}
BLOCK BB5 (BB6, BB7, ):
	1  il_seq_point il: 0x3
	1  move R19 <- R16
	GEN: R16(0)
	KILL: R19(3)
	1  lcompare_imm R16
	GEN: R16(0)
	1  long_beq [B6B7]
GEN  BB5: {0}
KILL BB5: {3}
BLOCK BB6 (BB9, ):
	1  il_seq_point il: 0xd
	1  iconst R28 <- [0]
	1  zext_i4 R18 <- R28
	KILL: R18(2)
	1  il_seq_point il: 0x10
	1  br [B9]
GEN  BB6: {}
KILL BB6: {2}
BLOCK BB9 (BB1, ):
	1  il_seq_point il: 0x1b
	1  long_add_imm R41 <- R18 [4] clobbers: 1
	GEN: R18(2)
	1  move R42 <- R18
	GEN: R18(2)
	1  move R43 <- R41
	1  voidcall [void P:Bar (int*,int*)] [%rdi <- R42] [%rsi <- R43] clobbers: c
	1  il_seq_point il: 0x2e, nonempty-stack
	1  il_seq_point il: 0x2e
	1  i8const R19 <- [0]
	KILL: R19(3)
	1  il_seq_point il: 0x30
	1  icompare_imm R17 [0]
	GEN: R17(1)
	1  int_ceq R48 <-
	1  move R49 <- R16
	GEN: R16(0)
	1  move R50 <- R48
	1  voidcall [void P:Plop (S[],bool)] [%rdi <- R49] [%rsi <- R50] clobbers: c
	1  il_seq_point il: 0x3a, nonempty-stack
	1  il_seq_point il: 0x3a
	1  br [B1]
GEN  BB9: {0, 1, 2}
KILL BB9: {3}
BLOCK BB7 (BB8, BB6, ):
	1  il_seq_point il: 0x8
	1  not_null R19
	GEN: R19(3)
	1  loadi4_membase R26 <- [R19 + 0x18]
	GEN: R19(3)
	1  not_null R19
	GEN: R19(3)
	1  icompare_imm R26 [0]
	1  int_bne_un [B8B6]
GEN  BB7: {3}
KILL BB7: {}
BLOCK BB8 (BB9, ):
	1  il_seq_point il: 0x12
	1  not_null R19
	GEN: R19(3)
	1  amd64_icompare_membase_imm [R19 + 0x18]
	GEN: R19(3)
	1  cond_exc_le_un IndexOutOfRangeException
	1  long_add_imm R18 <- R19 [32] clobbers: 1
	GEN: R19(3)
	KILL: R18(2)
GEN  BB8: {3}
KILL BB8: {2}

verbose-method-regalloc

Now that the store is eliminated, there's nothing left:

LOCAL REGALLOC BLOCK 9:
	1  il_seq_point il: 0x1b
	2  amd64_lea_membase R41 <- %r13
	3  move R42 <- %r13
	4  move R43 <- R41
	5  voidcall [void P:Bar (int*,int*)] [%rdi <- R42] [%rsi <- R43] clobbers: c
	6  il_seq_point il: 0x2e, nonempty-stack
	7  il_seq_point il: 0x2e
	8  il_seq_point il: 0x30
	9  loadu1_membase R59 <- [%rsp + 0x10]
	10 x86_test_null R59
	11 int_ceq R48 <-
	12 move R49 <- %r14
	13 move R50 <- R48
	14 voidcall [void P:Plop (S[],bool)] [%rdi <- R49] [%rsi <- R50] clobbers: c
	15 il_seq_point il: 0x3a, nonempty-stack
	16 il_seq_point il: 0x3a
	17 br [B1]