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Fix bad folding #54722

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Merged
sandreenko merged 2 commits into from
Jul 13, 2021
Merged

Fix bad folding #54722

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7e030a8
Always sign-extend from int32 in gtFoldExprConst
SingleAccretion Jun 25, 2021
576315f
Add a simple test that reproduces bad codegen
SingleAccretion Jun 25, 2021
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Always sign-extend from int32 in gtFoldExprConst 
GT_CNS_INT of TYP_INT on 64 hosts has an implicit contract:
the value returned by IconValue() must "fit" into 32 bit
signed integer, i. e. "static_cast<int>(IconValue()) == IconValue()".
gtFoldExprConst was failing to uphold this contract when the target
was 32 bit and the host was 64 bit.

Fix this by always truncating before calling SetIconValue().
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@SingleAccretion
SingleAccretion committed Jun 25, 2021
commit 7e030a88f7671b1d7893464b415c09e6c534be1b
12 changes: 4 additions & 8 deletions src/coreclr/jit/gentree.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -14864,19 +14864,15 @@ GenTree* Compiler::gtFoldExprConst(GenTree* tree)
JITDUMP("\nFolding operator with constant nodes into a constant:\n");
DISPTREE(tree);

#ifdef TARGET_64BIT
// Some operations are performed as 64 bit instead of 32 bit so the upper 32 bits
// need to be discarded. Since constant values are stored as ssize_t and the node
// has TYP_INT the result needs to be sign extended rather than zero extended.
i1 = INT32(i1);
#endif // TARGET_64BIT

// Also all conditional folding jumps here since the node hanging from
// GT_JTRUE has to be a GT_CNS_INT - value 0 or 1.

tree->ChangeOperConst(GT_CNS_INT);
tree->ChangeType(TYP_INT);
tree->AsIntCon()->SetIconValue(i1);
// Some operations are performed as 64 bit instead of 32 bit so the upper 32 bits
// need to be discarded. Since constant values are stored as ssize_t and the node
// has TYP_INT the result needs to be sign extended rather than zero extended.
tree->AsIntCon()->SetIconValue(static_cast<int>(i1));
tree->AsIntCon()->gtFieldSeq = fieldSeq;
if (vnStore != nullptr)
{
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