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128-bit addition on RV32 can introduce a conditional branch

00f.net 6 min read Editor score 7/10

The compiler splits 128-bit addition into 32-bit operations and propagates carry between them, producing a branch in the shown code. The example shows how source-level arithmetic can map unexpectedly onto low-level control flow.

What

- A compiler can introduce conditional branches when generating code for operations intended to be branch-free.

How

- On RV32, 128-bit addition is split into smaller additions with carry propagation; even a bit-mask selection compiled to a branch in the examples.

Why it matters

- Engineers writing constant-time code must inspect generated assembly and repeat the review when compiler versions or flags change.

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