--- /dev/null
+# Exception Handling Write Through Optimization.
+
+Write through is an optimization done on local variables that live across exception handling flow like a handler, filter, or finally so that they can be enregistered - treated as a register candidate - throughout a method. For each variable live across one of these constructs, the minimum requirement is that a store to the variables location on the stack is placed between a reaching definition and any point of control flow leading to the handler, as well as a load between any return from a filter or finally and an upward exposed use. Conceptually this maintains the value of the variable on the stack across the exceptional flow which would kill any live registers. This transformation splits a local variable into multiple enregisterable compiler temporaries backed by the local variable on the stack. For local vars that additionally have appearances within a eh construct, a load from the stack local is inserted to a temp that will be enregistered within the handler.
+
+## Motivation
+
+Historically the JIT has not done this transformation because exception handling was rare and thus the transformation was not worth the compile time. Additionally it was easy to make the recomendation to users to remove EH from performance critical methods since they had control of where the EH appeared. Neither of these points remain true as we increase our focus on cloud workloads. The use of non-blocking async calls are common in performance critical paths for these workloads and async injects exception handling constructs to implement the feature. This in combination with the long standing use of EH in 'foreach' and 'using' statements means that we are seeing EH constructs that are difficult for the user to manage or remove high in the profile (Techempower on Kestrel is a good example). Given these cloud workloads doing the transformation would be a clear benefit.
+
+## Design
+
+The goal of the design is to preserve the constraints listed above - i.e. preserve a correct value on the stack for any local var that crosses an EH edge in the flow graph. To ensure that the broad set of global optimizations can act on the IR shape produced by this transformation and that phase ordering issues do not block enregistration opportunities the write through phase will be staged just prior to SSA build after morph and it will do a full walk of the IR rewriting appearances to proxies as well as inserting reloads at the appropriate blocks in the flow graph as indicated by EH control flow semantics. To preserve the needed values on the stack a store will also be inserted after every definition to copy the new value in the proxy back to the stack location. This will leave non optimal number of stores (too many) but with the strategy that the more expensive analysis to eliminate/better place stores will be staged as a global optimization in a higher compilation tier.
+
+### Throughput
+
+To identify EH crossing local vars global liveness is necessary. This comes at the significant cost of the liveness analysis. To mitigate this the write through phase is staged immediately before SSA build for the global optimizer. Since the typical case is that there is no EH, the liveness analysis in write through can be reused directly by SSA build. For the case where EH local vars are present liveness today must be rebuilt for SSA since new local vars have been added, but incremental update to the RyuJIT liveness analysis can be implemented (worklist based live analysis) to improve the throughput. Additionally the write through transformation does a full IR walk - also expensive - to replace EH local var appearances with proxies and insert transfers to and from the stack for EH flow, given this initial implementations may need to be staged as part of AOT (crossgen) compiles until tiering can move the more expensive analysis out of the startup path.
+
+### Algorithm
+On the IR directly before SSA build:
+- Run global liveness to identify local vars that cross EH boundaries (as a byproduct of this these local vars are marked "do not enregister")
+- Foreach EH local var create a new local var "proxy" that can be enregisterd.
+- Iterate each block in the flow graph doing the following:
+ * Foreach tree in block do a post order traversal and
+ - Replace all appearances of EH local vars with the defined proxy
+ - Insert a copy of proxy definition back to the EH local var (on the stack)
+ * If EH handler entry block insert reloads from EH local var to proxy at block head
+ * If finally or filter exit, insert reloads from EH local var to proxy at successor block heads
+- For method entry block, insert reloads from parameter EH local vars to proxies
+
+At end no proxy should be live across EH flow and all value updates will be written back to the stack location.
+
+## Next steps
+
+The initial prototype that produced the example bellow is currently being improved to make it production ready. At the same time a more extensive suite of example tests are being developed.
+
+- [X] Proof of concept prototype.
+- [ ] Production implementation of WriteThru phase.
+- [ ] Suite of optimization examples/regression tests.
+- [ ] Testing
+ * [ ] Full CI test pass.
+ * [ ] JIT benchmark diffs.
+ * [ ] Kestrel techempower numbers.
+
+## Example
+
+The following is a simple example that shows enregistration for a local var live, and modified, through a catch.
+
+#### Source code snippet
+
+```
+class Enreg01
+{
+ int val;
+ double dist;
+
+ public Enreg01(int x) {
+ val = x;
+ dist = (double)x;
+ }
+
+ [MethodImpl(MethodImplOptions.NoInlining)]
+ public int foo(ref double d) { return (int)d; }
+
+ [MethodImpl(MethodImplOptions.NoInlining)]
+ public int Run()
+ {
+ int sum = val;
+
+ try {
+ TryValue(97);
+ }
+ catch (ValueException e)
+ {
+ Console.WriteLine("Catching {0}", Convert.ToString(e.x));
+ sum += val + e.x;
+ foo(ref dist);
+ sum += val;
+ }
+
+ return sum;
+ }
+
+ [MethodImpl(MethodImplOptions.NoInlining)]
+ public int TryValue(int y)
+ {
+ if (y == 97)
+ {
+ Console.WriteLine("Throwing 97");
+ throw new ValueException(97);
+ }
+ else
+ {
+ return y;
+ }
+ }
+}
+```
+#### Post WriteThru GenTree nodes for Run() method
+
+The Run() contains the catch and is the only method the EH WriteThru modifies.
+
+```
+Creating enregisterable proxies:
+lvaGrabTemp returning 8 (V08 tmp5) (a long lifetime temp) called for Add proxy for EH Write Thru..
+Creating proxy V08 for local var V00
+
+lvaGrabTemp returning 9 (V09 tmp6) (a long lifetime temp) called for Add proxy for EH Write Thru..
+Creating proxy V09 for local var V01
+
+Trees after EH Write Thru
+
+---------------------------------------------------------------------------------------------------------------------------
+BBnum descAddr ref try hnd preds weight [IL range] [jump] [EH region] [flags]
+---------------------------------------------------------------------------------------------------------------------------
+BB01 [00000263A1C161B8] 1 1 [000..007) i label target
+BB02 [00000263A1C162D0] 1 0 BB01 1 [007..012) T0 try { } keep i try label gcsafe
+BB03 [00000263A1C16500] 2 BB02,BB04 1 [050..052) (return) i label target gcsafe
+++++ funclets follow
+BB04 [00000263A1C163E8] 0 0 0 [012..050)-> BB03 ( cret ) H0 F catch { } keep i rare label target gcsafe flet
+-------------------------------------------------------------------------------------------------------------------------------------
+
+------------ BB01 [000..007), preds={} succs={BB02}
+
+***** BB01, stmt 1
+ ( 3, 3) [000123] ------------ * stmtExpr void (IL ???... ???)
+N001 ( 3, 2) [000120] ------------ | /--* lclVar ref V00 this
+N003 ( 3, 3) [000122] -A------R--- \--* = ref
+N002 ( 1, 1) [000121] D------N---- \--* lclVar ref V08 tmp5
+
+***** BB01, stmt 2
+ ( 17, 13) [000005] ------------ * stmtExpr void (IL 0x000...0x006)
+N007 ( 3, 2) [000097] ------------ | /--* lclVar int V09 tmp6
+N009 ( 7, 5) [000098] -A------R--- | /--* = int
+N008 ( 3, 2) [000096] D------N---- | | \--* lclVar int V01 loc0
+N010 ( 17, 13) [000099] -A-XG------- \--* comma void
+N004 ( 6, 5) [000002] ---XG------- | /--* indir int
+N002 ( 1, 1) [000059] ------------ | | | /--* const long 16 field offset Fseq[val]
+N003 ( 4, 3) [000060] -------N---- | | \--* + byref
+N001 ( 3, 2) [000001] ------------ | | \--* lclVar ref V08 tmp5
+N006 ( 10, 8) [000004] -A-XG---R--- \--* = int
+N005 ( 3, 2) [000003] D------N---- \--* lclVar int V09 tmp6
+
+------------ BB02 [007..012), preds={BB01} succs={BB03}
+
+***** BB02, stmt 3
+ ( 16, 10) [000013] ------------ * stmtExpr void (IL 0x007...0x00F)
+N008 ( 16, 10) [000011] --C-G------- \--* call int Enreg01.TryIncrement
+N004 ( 1, 1) [000009] ------------ this in rcx +--* lclVar ref V08 tmp5
+N005 ( 1, 1) [000010] ------------ arg1 in rdx \--* const int 97
+
+------------ BB03 [050..052) (return), preds={BB02,BB04} succs={}
+
+***** BB03, stmt 4
+ ( 3, 3) [000119] ------------ * stmtExpr void (IL ???... ???)
+N001 ( 3, 2) [000116] ------------ | /--* lclVar int V01 loc0
+N003 ( 3, 3) [000118] -A------R--- \--* = int
+N002 ( 1, 1) [000117] D------N---- \--* lclVar int V09 tmp6
+
+***** BB03, stmt 5
+ ( 4, 3) [000017] ------------ * stmtExpr void (IL 0x050...0x051)
+N002 ( 4, 3) [000016] ------------ \--* return int
+N001 ( 3, 2) [000015] ------------ \--* lclVar int V09 tmp6
+
+------------ BB04 [012..050) -> BB03 (cret), preds={} succs={BB03}
+
+***** BB04, stmt 6
+ ( 5, 4) [000021] ------------ * stmtExpr void (IL 0x012...0x012)
+N001 ( 1, 1) [000007] -----O------ | /--* catchArg ref
+N003 ( 5, 4) [000020] -A---O--R--- \--* = ref
+N002 ( 3, 2) [000019] D------N---- \--* lclVar ref V03 tmp0
+
+***** BB04, stmt 7
+ ( 3, 3) [000111] ------------ * stmtExpr void (IL ???... ???)
+N001 ( 3, 2) [000108] ------------ | /--* lclVar ref V00 this
+N003 ( 3, 3) [000110] -A------R--- \--* = ref
+N002 ( 1, 1) [000109] D------N---- \--* lclVar ref V08 tmp5
+
+***** BB04, stmt 8
+ ( 3, 3) [000115] ------------ * stmtExpr void (IL ???... ???)
+N001 ( 3, 2) [000112] ------------ | /--* lclVar int V01 loc0
+N003 ( 3, 3) [000114] -A------R--- \--* = int
+N002 ( 1, 1) [000113] D------N---- \--* lclVar int V09 tmp6
+
+***** BB04, stmt 9
+ ( 59, 43) [000034] ------------ * stmtExpr void (IL 0x013...0x037)
+N021 ( 59, 43) [000031] --CXG------- \--* call void System.Console.WriteLine
+N002 ( 5, 12) [000066] ----G------- | /--* indir ref
+N001 ( 3, 10) [000065] ------------ | | \--* const(h) long 0xB3963070 "Catching {0}"
+N004 ( 9, 15) [000076] -A--G---R-L- arg0 SETUP +--* = ref
+N003 ( 3, 2) [000075] D------N---- | \--* lclVar ref V05 tmp2
+N012 ( 20, 14) [000029] --CXG------- | /--* call ref System.Convert.ToString
+N010 ( 6, 8) [000028] ---XG------- arg0 in rcx | | \--* indir int
+N008 ( 1, 4) [000067] ------------ | | | /--* const long 140 field offset Fseq[x]
+N009 ( 4, 6) [000068] -------N---- | | \--* + byref
+N007 ( 3, 2) [000027] ------------ | | \--* lclVar ref V03 tmp0
+N014 ( 24, 17) [000072] -ACXG---R-L- arg1 SETUP +--* = ref
+N013 ( 3, 2) [000071] D------N---- | \--* lclVar ref V04 tmp1
+N017 ( 3, 2) [000073] ------------ arg1 in rdx +--* lclVar ref V04 tmp1 (last use)
+N018 ( 3, 2) [000077] ------------ arg0 in rcx \--* lclVar ref V05 tmp2 (last use)
+
+***** BB04, stmt 10
+ ( 18, 19) [000044] ------------ * stmtExpr void (IL 0x028... ???)
+N014 ( 1, 1) [000101] ------------ | /--* lclVar int V09 tmp6
+N016 ( 5, 4) [000102] -A------R--- | /--* = int
+N015 ( 3, 2) [000100] D------N---- | | \--* lclVar int V01 loc0
+N017 ( 18, 19) [000103] -A-XG------- \--* comma void
+N010 ( 6, 8) [000039] ---XG------- | /--* indir int
+N008 ( 1, 4) [000081] ------------ | | | /--* const long 140 field offset Fseq[x]
+N009 ( 4, 6) [000082] -------N---- | | \--* + byref
+N007 ( 3, 2) [000038] ------------ | | \--* lclVar ref V03 tmp0 (last use)
+N011 ( 13, 15) [000041] ---XG------- | /--* + int
+N005 ( 4, 4) [000037] ---XG------- | | | /--* indir int
+N003 ( 1, 1) [000079] ------------ | | | | | /--* const long 16 field offset Fseq[val]
+N004 ( 2, 2) [000080] -------N---- | | | | \--* + byref
+N002 ( 1, 1) [000036] ------------ | | | | \--* lclVar ref V08 tmp5
+N006 ( 6, 6) [000040] ---XG------- | | \--* + int
+N001 ( 1, 1) [000035] ------------ | | \--* lclVar int V09 tmp6
+N013 ( 13, 15) [000043] -A-XG---R--- \--* = int
+N012 ( 1, 1) [000042] D------N---- \--* lclVar int V09 tmp6
+
+***** BB04, stmt 11
+ ( 20, 14) [000051] ------------ * stmtExpr void (IL 0x038...0x044)
+N013 ( 20, 14) [000049] --CXGO------ \--* call int Enreg01.foo
+N007 ( 1, 1) [000086] ------------ | /--* const long 8 field offset Fseq[dist]
+N008 ( 3, 3) [000087] ------------ | /--* + byref
+N006 ( 1, 1) [000085] ------------ | | \--* lclVar ref V08 tmp5
+N009 ( 5, 5) [000088] ---XGO-N---- arg1 in rdx +--* comma byref
+N005 ( 2, 2) [000084] ---X-O-N---- | \--* nullcheck byte
+N004 ( 1, 1) [000083] ------------ | \--* lclVar ref V08 tmp5
+N010 ( 1, 1) [000045] ------------ this in rcx \--* lclVar ref V08 tmp5
+
+***** BB04, stmt 12
+ ( 11, 10) [000058] ------------ * stmtExpr void (IL 0x045...0x04D)
+N009 ( 1, 1) [000105] ------------ | /--* lclVar int V09 tmp6
+N011 ( 5, 4) [000106] -A------R--- | /--* = int
+N010 ( 3, 2) [000104] D------N---- | | \--* lclVar int V01 loc0
+N012 ( 11, 10) [000107] -A-XG------- \--* comma void
+N005 ( 4, 4) [000054] ---XG------- | /--* indir int
+N003 ( 1, 1) [000094] ------------ | | | /--* const long 16 field offset Fseq[val]
+N004 ( 2, 2) [000095] -------N---- | | \--* + byref
+N002 ( 1, 1) [000053] ------------ | | \--* lclVar ref V08 tmp5
+N006 ( 6, 6) [000055] ---XG------- | /--* + int
+N001 ( 1, 1) [000052] ------------ | | \--* lclVar int V09 tmp6
+N008 ( 6, 6) [000057] -A-XG---R--- \--* = int
+N007 ( 1, 1) [000056] D------N---- \--* lclVar int V09 tmp6
+
+```
+
+#### Post register allocation and code generation code
+
+```diff
+--- base.asmdmp 2017-03-28 20:40:36.000000000 -0700
++++ wt.asmdmp 2017-03-28 20:41:11.000000000 -0700
+@@ -1,78 +1,85 @@
+ *************** After end code gen, before unwindEmit()
+-G_M16307_IG01: ; func=00, offs=000000H, size=0014H, gcVars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, gcvars, byref, nogc <-- Prolog IG
++G_M16307_IG01: ; func=00, offs=000000H, size=0017H, gcVars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, gcvars, byref, nogc <-- Prolog IG
+
+ push rbp
++push r14
+ push rdi
+ push rsi
++push rbx
+ sub rsp, 48
+-lea rbp, [rsp+40H]
+-mov qword ptr [V07 rbp-20H], rsp
++lea rbp, [rsp+50H]
++mov qword ptr [V07 rbp-30H], rsp
+ mov gword ptr [V00 rbp+10H], rcx
+
+-G_M16307_IG02: ; offs=000014H, size=000AH, gcVars=0000000000000001 {V00}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, gcvars, byref
++G_M16307_IG02: ; offs=000017H, size=000AH, gcVars=0000000000000001 {V00}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, gcvars, byref
+
+-mov rcx, gword ptr [V00 rbp+10H]
+-mov ecx, dword ptr [rcx+16]
+-mov dword ptr [V01 rbp-14H], ecx
++mov rsi, gword ptr [V00 rbp+10H]
++mov edi, dword ptr [rsi+16]
++mov dword ptr [V01 rbp-24H], edi
+
+-G_M16307_IG03: ; offs=00001EH, size=000FH, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, byref
++G_M16307_IG03: ; offs=000021H, size=000EH, gcrefRegs=00000040 {rsi}, byrefRegs=00000000 {}, byref
+
+-mov rcx, gword ptr [V00 rbp+10H]
++mov rcx, rsi ; Elided reload in try region
+ mov edx, 97
+ call Enreg01:TryIncrement(int):int:this
+ nop
+
+-G_M16307_IG04: ; offs=00002DH, size=0003H, gcVars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, gcvars, byref
++G_M16307_IG04: ; offs=00002FH, size=0005H, gcVars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}, gcvars, byref
+
+-mov eax, dword ptr [V01 rbp-14H]
++mov edi, dword ptr [V01 rbp-24H]
++mov eax, edi
+
+-G_M16307_IG05: ; offs=000030H, size=0008H, epilog, nogc, emitadd
++G_M16307_IG05: ; offs=000034H, size=000BH, epilog, nogc, emitadd
+
+-lea rsp, [rbp-10H]
++lea rsp, [rbp-20H]
++pop rbx
+ pop rsi
+ pop rdi
++pop r14
+ pop rbp
+ ret
+
+-G_M16307_IG06: ; func=01, offs=000038H, size=0014H, gcrefRegs=00000004 {rdx}, byrefRegs=00000000 {}, byref, funclet prolog, nogc
++G_M16307_IG06: ; func=01, offs=00003FH, size=0017H, gcrefRegs=00000004 {rdx}, byrefRegs=00000000 {}, byref, funclet prolog, nogc
+
+ push rbp
++push r14
+ push rdi
+ push rsi
++push rbx
+ sub rsp, 48
+ mov rbp, qword ptr [rcx+32]
+ mov qword ptr [rsp+20H], rbp
+-lea rbp, [rbp+40H]
++lea rbp, [rbp+50H]
+
+-G_M16307_IG07: ; offs=00004CH, size=005EH, gcVars=0000000000000001 {V00}, gcrefRegs=00000004 {rdx}, byrefRegs=00000000 {}, gcvars, byref, isz
++G_M16307_IG07: ; offs=000056H, size=0054H, gcVars=0000000000000001 {V00}, gcrefRegs=00000004 {rdx}, byrefRegs=00000000 {}, gcvars, byref, isz
+
+ mov rsi, rdx
+-mov rcx, 0x18A3C473070
+-mov rdi, gword ptr [rcx]
++mov rcx, gword ptr [V00 rbp+10H] ; Reload of proxy register
++mov rdi, rcx ; Missed peep
++mov ecx, dword ptr [V01 rbp-24H] ; Reload of proxy register
++mov ebx, ecx ; Missed peep
++mov rcx, 0x263B3963070
++mov r14, gword ptr [rcx] ; Missed addressing mode
+ mov ecx, dword ptr [rsi+140]
+ call System.Convert:ToString(int):ref
+ mov rdx, rax
+-mov rcx, rdi
++mov rcx, r14
+ call System.Console:WriteLine(ref,ref)
+-mov edx, dword ptr [V01 rbp-14H] ; Elided stack access
+-mov rcx, gword ptr [V00 rbp+10H] ; Elided stack access
+-add edx, dword ptr [rcx+16]
+-add edx, dword ptr [rsi+140]
+-mov dword ptr [V01 rbp-14H], edx ; Elided stack access
+-mov rdx, gword ptr [V00 rbp+10H] ; Elided stack access
+-add rdx, 8
+-mov rcx, gword ptr [V00 rbp+10H] ; Elided stack access
++add ebx, dword ptr [rdi+16]
++add ebx, dword ptr [rsi+140]
++lea rdx, bword ptr [rdi+8]
++mov rcx, rdi
+ call Enreg01:foo(byref):int:this
+-mov eax, dword ptr [V01 rbp-14H] ; Elided stack access
+-mov rdx, gword ptr [V00 rbp+10H] ; Elided stack access
+-add eax, dword ptr [rdx+16]
+-mov dword ptr [V01 rbp-14H], eax ; Elided stack access
++add ebx, dword ptr [rdi+16]
++mov dword ptr [V01 rbp-24H], ebx ; Store of proxy register
+ lea rax, G_M16307_IG04
+
+-G_M16307_IG08: ; offs=0000AAH, size=0008H, funclet epilog, nogc, emitadd
++G_M16307_IG08: ; offs=0000AAH, size=000BH, funclet epilog, nogc, emitadd
+
+ add rsp, 48
++pop rbx
+ pop rsi
+ pop rdi
++pop r14
+ pop rbp
+ ret
+
+```
+
+Summary of diff:
+replaced 6 loads and 2 stores with 2 loads, 1 store, 2 push, 2 pop.
+
+
+
projects / platform / upstream / coreclr.git / commitdiff