[vm/arm64] Avoid conditional branch to ret in identical() stub
This is attempt to workaround a possible CPU bug on Exynos 2100 SoC.
The fix is tentative as we were unable to locate an erratum which
matches the symptoms and the specific code pattern.
https://github.com/flutter/flutter/issues/88261
TEST=user manually verified the fix on a Exynos 2100 SoC device
Cq-Include-Trybots: luci.dart.try:vm-kernel-linux-release-simarm64-try,vm-kernel-precomp-linux-release-simarm64-try,vm-ffi-android-release-arm64-try
Change-Id: I3be20d47d710a6bc520f2f7d6da81483baf74787
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/211240
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
diff --git a/runtime/vm/compiler/stub_code_compiler_arm64.cc b/runtime/vm/compiler/stub_code_compiler_arm64.cc
index 2c1288d..8694eb4 100644
--- a/runtime/vm/compiler/stub_code_compiler_arm64.cc
+++ b/runtime/vm/compiler/stub_code_compiler_arm64.cc
@@ -3173,15 +3173,14 @@
}
// Does identical check (object references are equal or not equal) with special
-// checks for boxed numbers.
-// Left and right are pushed on stack.
-// Return Zero condition flag set if equal.
-// Note: A Mint cannot contain a value that would fit in Smi.
+// checks for boxed numbers and returns with ZF set iff left and right are
+// identical.
static void GenerateIdenticalWithNumberCheckStub(Assembler* assembler,
const Register left,
const Register right) {
- Label reference_compare, done, check_mint;
+ Label reference_compare, check_mint;
// If any of the arguments is Smi do reference compare.
+ // Note: A Mint cannot contain a value that would fit in Smi.
__ BranchIfSmi(left, &reference_compare);
__ BranchIfSmi(right, &reference_compare);
@@ -3189,27 +3188,31 @@
__ CompareClassId(left, kDoubleCid);
__ b(&check_mint, NE);
__ CompareClassId(right, kDoubleCid);
- __ b(&done, NE);
+ __ b(&reference_compare, NE); // Do not branch directly to ret! See below.
// Double values bitwise compare.
__ LoadFieldFromOffset(left, left, target::Double::value_offset());
__ LoadFieldFromOffset(right, right, target::Double::value_offset());
__ CompareRegisters(left, right);
- __ b(&done);
+ __ ret();
__ Bind(&check_mint);
__ CompareClassId(left, kMintCid);
__ b(&reference_compare, NE);
__ CompareClassId(right, kMintCid);
- __ b(&done, NE);
+ __ b(&reference_compare, NE); // Do not branch directly to ret! See below.
__ LoadFieldFromOffset(left, left, target::Mint::value_offset());
__ LoadFieldFromOffset(right, right, target::Mint::value_offset());
__ CompareRegisters(left, right);
- __ b(&done);
+ __ ret();
__ Bind(&reference_compare);
__ CompareObjectRegisters(left, right);
- __ Bind(&done);
+ // None of the branches above go directly here to avoid generating a conditional
+ // branch to a ret instruction.
+ // This is an attempt to work-around a possible CPU on Exynos 2100 SoC.
+ // See https://github.com/flutter/flutter/issues/88261
+ __ ret();
}
// Called only from unoptimized code. All relevant registers have been saved.
@@ -3235,7 +3238,6 @@
__ LoadFromOffset(left, SP, 1 * target::kWordSize);
__ LoadFromOffset(right, SP, 0 * target::kWordSize);
GenerateIdenticalWithNumberCheckStub(assembler, left, right);
- __ ret();
#if !defined(PRODUCT)
__ Bind(&stepping);
@@ -3259,7 +3261,6 @@
__ LoadFromOffset(left, SP, 1 * target::kWordSize);
__ LoadFromOffset(right, SP, 0 * target::kWordSize);
GenerateIdenticalWithNumberCheckStub(assembler, left, right);
- __ ret();
}
// Called from megamorphic call sites.
