What is the movzbl instruction and when does the compiler use it?

Question

Accepted Answer

"Move zero-extend byte to long": it reads one byte and fills the rest of the 32-bit destination with zeros.
# Read 1 byte from memory, fill upper bits with zeros
movzbl (%rax), %edx

If memory at (%rax) contains 0x41 ('A'), then %edx becomes 0x00000041.
The naming pattern: mov + z(zero) or s(sign) + source size + dest size

Instruction
Meaning

movzbl
Zero-extend byte → long (32-bit)

movzbq
Zero-extend byte → quad (64-bit)

movzwl
Zero-extend word → long

movsbl
Sign-extend byte → long

movslq
Sign-extend long → quad

Common context: Reading a char from a string, reading a byte from a lookup table, or widening a small value before arithmetic.
Go deeper:

MOVZX (zero-extend) reference — movzbl is MOVZX byte-to-long; the page shows DEST := ZeroExtend(SRC).
 x86 instruction listings (Wikipedia) — situates MOVZX/MOVSX among the data-movement instructions.

Instruction	Meaning
`movzbl`	Zero-extend byte → long (32-bit)
`movzbq`	Zero-extend byte → quad (64-bit)
`movzwl`	Zero-extend word → long
`movsbl`	Sign-extend byte → long
`movslq`	Sign-extend long → quad