[MS] Just for fun: Which processors prefer sign-extended loads, and which prefer zero-extended loads? - devamazonaws.blogspot.com

Just for fun, let's compare what happens on different processor architectures when you load a value from memory into a larger register. Possible responses are

  • Unable: The processor does not support loading data into a larger register.
  • Zero-extend: The processor zero-extends the small value to the large value.
  • Sign-extend: The processor sign-extends the small value to the large value.
  • Either: The processor has separate instructions for sign and zero extension.
  • (blank): Does not apply because the processor doesn't have registers larger than this.
Processor From byte From 16-bit From 32-bit
8086 Unable    
Alpha AXP Unable Unable Sign-extend
Alpha AXP with BWX Zero-extend Zero-extend Sign-extend
80386 Either Either  
x86-64 Either Either Either
Intel ia64 Zero-extend Zero-extend Zero-extend
MIPS Either Either Either
PowerPC Zero-extend Either Either
SH-4 Sign-extend Sign-extend  
ARM Either Either Either
68000 Unable Unable  
SPARC Either Either Either
RISC-V Either Either Either

Notes:

  • The Alpha AXP with BWX is the only processor on the list where the extension mode changes between zero and sign, depending on size. This is a historical artifact: It inherited the "sign-extend" entry from the "Alpha AXP" row above it, but it turns out that bytes are usually zero-extended, so the BWX extension chose zero-extension for bytes. Words went along with bytes to minimize the weirdness.
  • The x86-64 supports both zero-extended 32-to-64 loads and sign-extended 32-to-64 loads, but the zero-extended load offers a more compact encoding.

We can organize the processors by how they choose to extend small values to larger values:

Unable 8086, 68000, Alpha AXP (bytes and words)
Zero-extend ia64, PowerPC (bytes)
Sign-extend Alpha AXP, SH-4
Either 80386, x86-64, MIPS, ARM, SPARC, RISC-V
Mixed Alpha AXP with BWX, PowerPC

Post Updated on August 28, 2023 at 03:00PM
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