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JIT: have jit tail call stress avoid creating loops in some cases #73114

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Merged
AndyAyersMS merged 3 commits into from
Jul 31, 2022
Merged

JIT: have jit tail call stress avoid creating loops in some cases #73114

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f16b045
JIT: have jit stress create modifiable this earlier
AndyAyersMS Jul 30, 2022
972a2ff
Revert "JIT: have jit stress create modifiable this earlier"
AndyAyersMS Jul 30, 2022
5402e96
actual fix
AndyAyersMS Jul 30, 2022
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1 change: 1 addition & 0 deletions src/coreclr/jit/fgbasic.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2220,6 +2220,7 @@ void Compiler::fgAdjustForAddressExposedOrWrittenThis()
// Optionally enable adjustment during stress.
if (compStressCompile(STRESS_GENERIC_VARN, 15))
{
JITDUMP("JitStress: creating modifiable `this`\n");
thisVarDsc->lvHasILStoreOp = true;
}

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8 changes: 7 additions & 1 deletion src/coreclr/jit/importer.cpp
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Expand Up @@ -15665,7 +15665,13 @@ void Compiler::impImportBlockCode(BasicBlock* block)
constraintCall ? &constrainedResolvedToken : nullptr,
returnFalseIfInvalid);

if (passedConstraintCheck)
// Avoid setting compHasBackwardsJump = true via tail call stress if the method cannot have
// patchpoints.
//
const bool mayHavePatchpoints = opts.jitFlags->IsSet(JitFlags::JIT_FLAG_TIER0) &&
(JitConfig.TC_OnStackReplacement() > 0) &&
compCanHavePatchpoints();
if (passedConstraintCheck && (mayHavePatchpoints || compHasBackwardJump))
Comment on lines +15668 to +15674
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@jakobbotsch jakobbotsch Jul 31, 2022

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The fix looks fine to me but I am a bit confused we are marking a backward jump in the first place. We should not do the loop optimization in tier-0 (and, since #41059, never under tailcall stress).

I can clean this up in the future if you don't want to do this as part of this PR.

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@jakobbotsch jakobbotsch Jul 31, 2022

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But clearly you had to add special support for OSR due to the tailcall-to-loop optimization, so maybe I am missing something?

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@AndyAyersMS AndyAyersMS Jul 31, 2022

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No, you're on to something.

We are probably too aggressive in marking explicit tail calls as backwards branches in Tier0—and more importantly in this case, changing the value of compHasBackwardJump. We do this regardless of optimization flags. If we are never going to do the optimization, then we don't need to change compHasBackwardJump either.

Another possible fix would be that when tail call stress changes an implicit tail call to an explicit one, it should also set compTailPrefixSeen. But it's not as clean of a fix; we might already have added some patchpoints.

{
// Now check with the runtime
CORINFO_METHOD_HANDLE declaredCalleeHnd = callInfo.hMethod;
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