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Cranelift: miscompilation for shl.i128 with zero shift amount #2679

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bjorn3 opened this issue Feb 23, 2021 · 2 comments
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Cranelift: miscompilation for shl.i128 with zero shift amount #2679

bjorn3 opened this issue Feb 23, 2021 · 2 comments
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bug Incorrect behavior in the current implementation that needs fixing cranelift Issues related to the Cranelift code generator

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@bjorn3
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bjorn3 commented Feb 23, 2021 •
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Steps to Reproduce

  • Codegen 1u128 << 0 using ishl.i128

Expected Results

The resulting value is 1.

Actual Results

The resulting value is 0x10000000000000001.

Versions and Environment

Cranelift version or commit: 98d3e68

Operating system: N/A

Architecture: x86_64

Extra Info

This only happens for a zero shift amount. Any other amount is fine.
@bjorn3 bjorn3 added bug Incorrect behavior in the current implementation that needs fixing cranelift Issues related to the Cranelift code generator labels Feb 23, 2021
@bjorn3
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bjorn3 commented Feb 23, 2021

This is responsible for the miscompilation of https://github.com/bjorn3/rustc_codegen_cranelift/issues/1143.

@bjorn3 bjorn3 mentioned this issue Feb 23, 2021
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cfallin added a commit to cfallin/wasmtime that referenced this issue Feb 23, 2021
@cfallin
Fix some i128 shift-related bugs in x64 backend.
0f3e00b 
This fixes bytecodealliance#2672 and bytecodealliance#2679, and also fixes an incorrect instruction
emission (`test` with small immediate) that we had missed earlier.

The shift-related fixes have to do with (i) shifts by 0 bits, as a
special case that must be handled; and (ii) shifts by a 128-bit amount,
which we can handle by just dropping the upper half (we only use 3--7
bits of shift amount).

This adjusts the lowerings appropriately, and also adds run-tests to
ensure that the lowerings actually execute correctly (previously we only
had compile-tests with golden lowerings; I'd like to correct this for
more ops eventually, adding run-tests beyond what the Wasm spec and
frontend covers).
@cfallin cfallin mentioned this issue Feb 23, 2021
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cfallin commented Feb 26, 2021

Closed with #2682.

@cfallin cfallin closed this as completed Feb 26, 2021
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