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[wasm] Jiterpreter back branch optimizations #84067

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Mar 29, 2023
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[wasm] Jiterpreter back branch optimizations #84067

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88 changes: 58 additions & 30 deletions src/mono/wasm/runtime/jiterpreter-support.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1006,6 +1006,7 @@ class Cfg {
overheadBytes = 0;
entryBlob!: CfgBlob;
blockStack: Array<MintOpcodePtr> = [];
backDispatchOffsets: Array<MintOpcodePtr> = [];
dispatchTable = new Map<MintOpcodePtr, number>();
trace = 0;

Expand All @@ -1024,6 +1025,7 @@ class Cfg {
this.overheadBytes = 10; // epilogue
this.dispatchTable.clear();
this.trace = trace;
this.backDispatchOffsets.length = 0;
}

// We have a header containing the table of locals and we need to preserve it
Expand Down Expand Up @@ -1134,40 +1136,63 @@ class Cfg {

const dispatchIp = <MintOpcodePtr><any>0;
if (this.backBranchTargets) {
// the loop needs to start with a br_table that performs dispatch based on the current value
// of the dispatch index local
// br_table has to be surrounded by a block in order for a depth of 0 to be fallthrough
// We wrap it in an additional block so we can have a trap for unexpected disp values
this.builder.block(WasmValtype.void);
this.builder.block(WasmValtype.void);
this.builder.local("disp");
this.builder.appendU8(WasmOpcode.br_table);
// br_table <number of values starting from 0> <labels for values starting from 0> <default>
// we have to assign disp==0 to fallthrough so that we start at the top of the fn body, then
// assign disp values starting from 1 to branch targets
// FIXME: Only include back branch targets that are *also* in the block stack. This is necessary
// when starting a trace in the middle of a method to make the table smaller
this.builder.appendULeb(this.backBranchTargets.length + 1);
this.builder.appendULeb(1); // br depth of 1 = skip the unreachable and fall through to the start
this.backDispatchOffsets.length = 0;
// First scan the back branch target table and union it with the block stack
// This filters down to back branch targets that are reachable inside this trace
for (let i = 0; i < this.backBranchTargets.length; i++) {
const offset = (this.backBranchTargets[i] * 2) + <any>this.startOfBody;
const breakDepth = this.blockStack.indexOf(offset);
if (breakDepth >= 0) {
this.dispatchTable.set(offset, i + 1);
this.builder.appendULeb(breakDepth + 2); // add 2 to the depth because of the double block around it
} else {
// This means the back branch target is outside of the trace. It shouldn't be possible to reach this
// and we didn't add it to the dispatch table anyway
this.builder.appendULeb(0);
this.dispatchTable.set(offset, this.backDispatchOffsets.length + 1);
this.backDispatchOffsets.push(offset);
}
}
this.builder.appendULeb(0); // for unrecognized value we br 0, which causes us to trap
this.builder.endBlock();
this.builder.appendU8(WasmOpcode.unreachable);
this.builder.endBlock();
// We put a dummy IP at the end of the block stack to represent the dispatch loop
// We will use this dummy IP to find the appropriate br depth when restarting the loop later
this.blockStack.push(dispatchIp);

if (this.backDispatchOffsets.length === 0) {
if (this.trace > 0)
console.log("No back branch targets were reachable after filtering");
} else if (this.backDispatchOffsets.length === 1) {
if (this.trace > 0) {
if (this.backDispatchOffsets[0] === this.entryIp)
console.log(`Exactly one back dispatch offset and it was the entry point 0x${(<any>this.entryIp).toString(16)}`);
else
console.log(`Exactly one back dispatch offset and it was 0x${(<any>this.backDispatchOffsets[0]).toString(16)}`);
}

// if (disp) goto back_branch_target else fallthrough
this.builder.local("disp");
this.builder.appendU8(WasmOpcode.br_if);
this.builder.appendULeb(this.blockStack.indexOf(this.backDispatchOffsets[0]));
} else {
// the loop needs to start with a br_table that performs dispatch based on the current value
// of the dispatch index local
// br_table has to be surrounded by a block in order for a depth of 0 to be fallthrough
// We wrap it in an additional block so we can have a trap for unexpected disp values
this.builder.block(WasmValtype.void);
this.builder.block(WasmValtype.void);
this.builder.local("disp");
this.builder.appendU8(WasmOpcode.br_table);

// br_table <number of values starting from 0> <labels for values starting from 0> <default>
// we have to assign disp==0 to fallthrough so that we start at the top of the fn body, then
// assign disp values starting from 1 to branch targets
this.builder.appendULeb(this.backDispatchOffsets.length + 1);
this.builder.appendULeb(1); // br depth of 1 = skip the unreachable and fall through to the start
for (let i = 0; i < this.backDispatchOffsets.length; i++) {
// add 2 to the depth because of the double block around it
this.builder.appendULeb(this.blockStack.indexOf(this.backDispatchOffsets[i]) + 2);
}
this.builder.appendULeb(0); // for unrecognized value we br 0, which causes us to trap
this.builder.endBlock();
this.builder.appendU8(WasmOpcode.unreachable);
this.builder.endBlock();
}

if (this.backDispatchOffsets.length > 0) {
// We put a dummy IP at the end of the block stack to represent the dispatch loop
// We will use this dummy IP to find the appropriate br depth when restarting the loop later
this.blockStack.push(dispatchIp);
}
}

if (this.trace > 1)
Expand Down Expand Up @@ -1250,8 +1275,11 @@ class Cfg {

// Close the dispatch loop
if (this.backBranchTargets) {
mono_assert(this.blockStack[0] === <any>0, "expected one zero entry on the block stack for the dispatch loop");
this.blockStack.shift();
// This is no longer true due to filtering
// mono_assert(this.blockStack[0] === <any>0, "expected one zero entry on the block stack for the dispatch loop");
mono_assert(this.blockStack.length <= 1, "expected one or zero entries in the block stack at the end");
if (this.blockStack.length)
this.blockStack.shift();
this.builder.endBlock();
}

Expand Down