6 ms·

I learned C on the Amiga, back in the late 80's, and the OS made heavy use of "OO-ish" physical subtyping with structs everywhere. I don't think anybody even th

I learned C on the Amiga, back in the late 80's, and the OS made heavy use of "OO-ish" physical subtyping with structs everywhere. I don't think anybody even thought about strict aliasing violations.

ajross 2y ago
Compilers in the 1980's really weren't sophisticated enough to have this problem. A function call was a hard barrier that was going to spill all GPRs, inlining was almost unheard of. What the code did was what you saw, and if you had an aliased pointer it's because that's what you wanted. And when it became an issue c. late 90's, it was actually "NO strict aliasing" that was the point of contention. Optimizers were suddenly able to do all sorts of magic, and compiler authors realized they were getting tripped up by the inability (c.f. the halting problem) to know for sure that this arbitrary pointer wasn't scribbling over the memory contents they were trying to optimize. You'd get better (often much better) code with -fno-strict-aliasing, which was tempting enough to turn it on and hope for better analysis tools to come along and save us from the resulting bugs. We're still waiting, alas.
flohofwoe 2y ago
The Amiga C compilers most likely didn't do a lot of optimizations where strict aliasing would matter though (at least from what I remember it was pretty straight forward, a memory read or write in C typically resulted in a memory read or write in assembly). Basically, C code compiled to assembly in the Amiga era looked much more straightforward than the output produced by modern C compilers (with optimizations enabled at least), you could put both side by side and see a near 1:1 relationship between the C code and the assembly code (maybe also because the Motorola 68000 seems to have taken a lot of inspiration from the PDP instruction set).

6 ms·

I learned C on the Amiga, back in the late 80's, and the OS made heavy use of "OO-ish" physical subtyping with structs everywhere. I don't think anybody even th

by icedchai 2y ago

I learned C on the Amiga, back in the late 80's, and the OS made heavy use of "OO-ish" physical subtyping with structs everywhere. I don't think anybody even thought about strict aliasing violations.

ajross 2y ago
Compilers in the 1980's really weren't sophisticated enough to have this problem. A function call was a hard barrier that was going to spill all GPRs, inlining was almost unheard of. What the code did was what you saw, and if you had an aliased pointer it's because that's what you wanted. And when it became an issue c. late 90's, it was actually "NO strict aliasing" that was the point of contention. Optimizers were suddenly able to do all sorts of magic, and compiler authors realized they were getting tripped up by the inability (c.f. the halting problem) to know for sure that this arbitrary pointer wasn't scribbling over the memory contents they were trying to optimize. You'd get better (often much better) code with -fno-strict-aliasing, which was tempting enough to turn it on and hope for better analysis tools to come along and save us from the resulting bugs. We're still waiting, alas.
flohofwoe 2y ago
The Amiga C compilers most likely didn't do a lot of optimizations where strict aliasing would matter though (at least from what I remember it was pretty straight forward, a memory read or write in C typically resulted in a memory read or write in assembly). Basically, C code compiled to assembly in the Amiga era looked much more straightforward than the output produced by modern C compilers (with optimizations enabled at least), you could put both side by side and see a near 1:1 relationship between the C code and the assembly code (maybe also because the Motorola 68000 seems to have taken a lot of inspiration from the PDP instruction set).