https://bugs.llvm.org/show_bug.cgi?id=41094
Bug ID: 41094
Summary: want -fomit-frame-pointer by default on PowerPC
Product: clang
Version: trunk
Hardware: Macintosh
OS: OpenBSD
Status: NEW
Severity: enhancement
Priority: P
Component: Driver
Assignee: unassignedclangb...@nondot.org
Reporter: kern...@gmail.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
Created attachment 21609
--> https://bugs.llvm.org/attachment.cgi?id=21609&action=edit
default -fomit-frame-pointer for PowerPC
Right now, when clang targets PowerPC with optimizations, clang defaults to
-fomit-frame-pointer for Linux and NetBSD, but not for other targets. The
attached patch (for llvm-project.git master) changes the default to
-fomit-frame-pointer for all PowerPC targets.
I want this because I am targeting 32-bit PowerPC OpenBSD, where gcc already
uses -fomit-frame-pointer by default. All targets except Darwin have a similar
System V ELF ABI, so if -fomit-frame-pointer works for Linux and NetBSD, it
would probably work for FreeBSD and OpenBSD. (It might work for Darwin, but
"Darwin is no longer supported for PowerPC" in llvm's PPCTargetMachine.) The
frame pointer r31, when it exists, is almost always an extra copy of the stack
pointer r1.
Consider this code: void nothing(void) {}
`clang -target powerpc-openbsd -O3 -S nothing.c` now emits
stwu 1, -16(1) # allocate stack frame
stw 31, 12(1) # save old r31
mr 31, 1 # set frame pointer r31 = stack pointer r1
lwz 31, 12(1) # restore old r31
addi 1, 1, 16 # free stack frame
blr # return
With -fomit-frame-pointer or with the patch, clang emits only
blr
Beware that -fno-omit-frame-pointer doesn't work; this patch doesn't fix that.
In the near future, I will backport the patch to clang 7 and compile more code.
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