winch: Simplify constant handling, part 2/N #10989
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This commit introduces a register allocator for scratch registers. The objective of this change is to make it generally safer to work with scratch registers and prevent accidental clobbering of said registers. This approach also has the advantage that allows for a more natural abstraction over ISA-dependent scratch register definitions, e.g., we can easily encode that fact that in aarch64 x16 and x17 are considered scratch registers, while in x64 Winch's ABI defines a single global scratch register.
This commit makes use of the scratch register allocator in both the x64 and aarch64 backends for immediate value loading. Given that the MacroAssembler is the boundary between ISA-agnostic code and ISA-dependent code, it seems to be the natural location for this allocator to live. The allocator gives exclusive access to a scratch register of a particular class, through the `Masm::with_scratch` method. Note that the semantics of this allocator don't involve spilling or any other form of register availability resolution. If a register is requested and it's not available, this method will panic.
Even though the entire change doens't contain major funcitonal changes, a side effect of improving the constant handling in aarch64 is that we perform better instruction selection for instructions that deal with immediattes, improving the generated code in some cases.
alexcrichton
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Jun 9, 2025
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This commit is a follow-up to bytecodealliance#10989; it migrates all the uses of the scratch register to use `Masm::with_scratch`. This commit also updates the tests that can fail / are ignored, since many of the bugs were introduced by accidental clobbers to scratch registers. Even though this change doesn't introduce any functional changes, the disassembly changes in aarch64 are due to the usage of x17 as an extra scratch registers. The updates in x64 dissembly are related to offset changes.
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This commit is a follow-up to bytecodealliance#10989; it migrates all the uses of the scratch register to use `Masm::with_scratch`. This commit also updates the tests that can fail / are ignored, since many of the bugs were introduced by accidental clobbers to scratch registers. Even though this change doesn't introduce any functional changes, the disassembly changes in aarch64 are due to the usage of x17 as an extra scratch registers. The updates in x64 dissembly are related to offset changes.
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* winch: Use `Masm::with_scratch` in ISA-agnostic code This commit is a follow-up to #10989; it migrates all the uses of the scratch register to use `Masm::with_scratch`. This commit also updates the tests that can fail / are ignored, since many of the bugs were introduced by accidental clobbers to scratch registers. Even though this change doesn't introduce any functional changes, the disassembly changes in aarch64 are due to the usage of x17 as an extra scratch registers. The updates in x64 dissembly are related to offset changes. * Use `with_scratch!` in `pop_to_addr` * Remove dead code * Apply cargo fmt * Remove `imports.wast` from ignore list * Use the new version of `Masm::with_scratch` * Introduce `Masm::with_scratch_for` Helper to derive the register class needed for a particular Wasm value type * Format
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This patch is best reviewed commit by commit.
This is a follow-up to #10888
Scratch registers are used throughout Winch's codebase to handle constant values that cannot be directly encoded in specific instructions. Since the encoding rules are ISA-specific, selecting instructions for these operations often requires scratch registers. Ideally, these temporary registers should be acquired without adding any register pressure to prevent value spilling.
The objective of this change is to make it generally safer to work with scratch registers and prevent accidental clobbering of such registers.