Disassemble and Match C to Assembly

Topic Overview

Compiler Explorer in Your Terminal

Matching C to assembly is the daily work of performance and security engineers. gcc -S emits what Clang would execute; objdump -d -S interleaves source when debug symbols exist. The gap between -O0 and -O3 is the gap between naive loops and vectorized kernels.

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Typical lowering patterns:

• int x = 5; → mov DWORD PTR [rbp-4], 5
• if (a > b) return a; → cmp + conditional mov + ret
• for (i = 0; i < n; i++) → counter in a register or stack slot, jl back

For this exercise, you will write add, max, and sum_array in C, compile to .s, and annotate the emitted instructions. This task asks you to label which C line produced each block, because "the compiler is slow" tickets become fixable once you can read the hot loop it actually emitted.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB. Skim the official documentation for the tool or ABI named in the exercise; the prose changes, but register roles, syscall numbers, and ownership rules stay stable across releases.

Compute a compiled expression step by step, the way the emitted assembly would.

Input is three 32-bit signed integers a, b, c. The compiled body being simulated is:

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All arithmetic is 32-bit two's complement with wraparound. Print each intermediate value as a signed 32-bit result. The sar step is the heart of the exercise: arithmetic right shift sign-fills, so (-5) >> 2 is -2 (floor division), not C's truncating -5 / 4 = -1.

Requirements:

• Implement sar with sign extension for negative inputs (shift the unsigned pattern, then fill top bits with the sign bit)
• shl, xor, add, and the lea multiply operate on the 32-bit pattern; v wraps mod 2^32
• Print every intermediate as a signed 32-bit value
• No undefined behavior: do the bit work in unsigned integers