[cfe-dev] [llvm-dev] difference between --target, -mcpu, -march
Krzysztof Parzyszek via cfe-dev
cfe-dev at lists.llvm.org
Sat Mar 19 09:08:11 PDT 2016
On 3/19/2016 5:54 AM, Bruce Hoult via llvm-dev wrote:
> Would it be fair to say that an -march chooses an instruction set, while
> -mcpu will affect scheduling things such as how many instructions can be
> run in parallel and their latency etc?
From what I remember, the meaning of march/mcpu differs between clang
and LLVM's utilities (llc, etc.) In clang, mcpu and march have a lot of
overlap in terms of usage, and it seems to differ from one target to the
next. What you say would make sense, but, for example, we have legacy
code that used -march=hexagonv5, where "hexagonv5" is actually a CPU
version, something that intuitively should belong in "mcpu". Hexagon
doesn't seem to be the only one where "march" and "mcpu" are treated
almost identically, I saw what looked like CPU names in the march string
in other toolchains as well.
The clang documentation states:
Target Selection Options
.. option:: -arch <architecture>
Specify the architecture to build for.
.. option:: -march=<cpu>
Specify that Clang should generate code for a specific processor
member and later. For example, if you specify -march=i486, the
allowed to generate instructions that are valid on i486 and later
but which may not exist on earlier ones.
mcpu is not mentioned at all.
For better or worse, clang is trying to emulate GCC's options, which
have a long history behind them. There is some general idea there, but
I think the target/processor selection scheme has evolved beyond the
point where it makes sense on all levels.
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