Clang i386/i486/i586/i686 targets inaccurate

[Fierelier]

I have been trying to discuss the definition of i686 for Rust onto Pentium Pro, see my reasoning here: rust-lang/rust#82435 (comment) -- In the answer to this, I was told that clang implies Pentium 4 for i686. See their godbolt: https://llvm.godbolt.org/z/PP69efvfx -- This inconsistency is one of the reasons keeping the Rust team from changing the definition.

For the same reasoning as in my post, I think i686 should imply Pentium Pro, not Pentium 4. Clang is seemingly aware of this (See: https://clang.llvm.org/doxygen/Basic_2Targets_2X86_8cpp_source.html ), but it still uses a Pentium 4 cpu-target in the end for some reason.

[llvmbot]

@llvm/issue-subscribers-backend-x86

[topperc]

In LLVM and Clang i686 in the triple is treated the same as i586, i486, and i386. See this code in Triple.cpp

static Triple::ArchType parseArch(StringRef ArchName) {                          
  auto AT = StringSwitch<Triple::ArchType>(ArchName)                             
    .Cases("i386", "i486", "i586", "i686", Triple::x86) 

They all mean 32-bit x86.

The selection of "pentium4" is done by this code

llvm-project/clang/lib/Driver/ToolChains/Arch/X86.cpp

Line 115 in 9883558

return "pentium4";

based on OS being Linux and triple being 32-bit.

There is a separate -march=i686 option that can override the default.

[Fierelier]

Why isn't the triple more accurate? Why override the CPU at all?

EDIT: Actually, perhaps I should contact the rust devs instead, since I suspect distros would set the appropriate flags if they can? This is a big mess... Having this set straight would help regardless though, I'm sure.

[topperc]

Why isn't the triple more accurate? Why override the CPU at all?

EDIT: Actually, perhaps I should contact the rust devs instead, since I suspect distros would set the appropriate flags if they can? This is a big mess... Having this set straight would help regardless though, I'm sure.

The first part of the triple isn't a CPU, it's an architecture. We don't allow arbitrary CPUs there. I think it comes from GNU tools. gcc's autoconf for building uses triples I don't know if gcc makes a distinction between i386, i486, i586, i686 in autoconf.

[Fierelier]

Right, I'm aware it isn't a CPU, it's just that LLVM compiles for the Pentium 4 CPU if i686 is chosen, like you've said, which is not the minimum for i686 (Pentium Pro), which should be targeted instead in my opinion because.. Let's take a modern-day example of an x86-64 CPU:
I compile binaries on my intel 8000 series CPU, and a few years later the 13000 series comes out. Suddenly the x86-64 spec has been upgraded to include new instruction sets, and binaries compiled in x86-64 suddenly no longer work on my intel 8000 series -- That would be really annoying

Text in cursive is theoretical, not an actual issue. Assuming x86-64 doesn't do that, why should i686? https://en.wikipedia.org/wiki/P6_(microarchitecture)

[topperc]

Clang also uses pentium4 for Linux if the triple is i386, i486, or i586. They're all treated as meaning 32-bit. What is the default CPU for gcc when built for i686 or when compiling with -m32?

[Fierelier]

The only thing I could find was this:
https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html
"The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs on any i386 system."

[Fierelier]

I think GCC thinks of i686-*-* and even i786-*-* as Pentium Pro: https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config.gcc;h=f986224817a12888e51be3873b6f230ea900bccb;hb=HEAD#l3789

[phoebewang]

How about just make i686- and i786 override the OS part? https://reviews.llvm.org/D145857

[Fierelier]

I think that looks in-line with gcc for i686/i786, yes. But as said in #61347 (comment) this issue also affects i386, i486 and i586. I'm not sure what llvm compiles down to (and in the case of Linux, i486 might get dropped if it hasn't already). Either way, this looks like a good first step to me, at the very least.