https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116497
Bug ID: 116497
Summary: Need no_caller_saved_registers with SSE support
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: andi-gcc at firstfloor dot org
CC: hjl.tools at gmail dot com
Target Milestone: ---
Target: x86_64-linux
When writing threaded code interpreters by chaining functions with musttail the
normal ABI behavior of some caller saved registers can cause unnecessary spills
and fills compared to using indirect goto.
In principle this could be avoided by using no_caller_saved_registers on the
musttail called function, and perhaps no_callee_saved_registers on the function
that starts the interpretation chain to maintain the ABI on the interpreter
entry point.
But these attributes were designed for interrupt handlers and require disabling
SSE because an interrupt handler needs to really preserve all registers. While
for the interpreter case which interacts with the normal ABI it is fine to
clobber the SSE registers, as specified by the x86_64 SYSV ABI
So disabling SSE can be done (and it is done in some real code today, see [1]
below) it is very inconvenient for an interpreter that may want to use SSE for
floating point etc.
So what we really need for the efficient musttail interpreters is
no_caller_saved_registers, but allow using SSE.
clang has a special ABI for this case (preserve_most[2]), but that seems
overkill.
There are two ways around this:
- We just remove the code that enforces no SSE for (see below patch). The
interrupt handlers would need to disable SSE without error and trust that
it doesn't happen by mistake.
I'm not sure there is much code that uses this (I couldn't find any).
Presumably they would rather use the interrupt attribute anyways.
- We define a new attribute like no_caller_saved_registers except that it
allows using SSE.
[1]
https://github.com/swoole/swoole-cli/blob/94ab97fbcfe39be8f5a985da82575bfb4c2319db/Zend/zend_string.c#L376
[2]
https://clang.llvm.org/docs/AttributeReference.html#preserve-most
