http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49583
Summary: Reloading stack operands in the wrong order, so needs
to insert fxch
Product: gcc
Version: 4.6.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: sgunder...@bigfoot.com
Created attachment 24638
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24638
Minimal testcase
Hi,
It seems that when generating x87 code, GCC sometimes reloads items from the
stack in the wrong order, and then goes to great lengths to swap them around. I
have an example with six loads and six fxch instructions, but attached is a
minimal example. Compiling with "gcc version 4.6.1 (Debian 4.6.1-1)" as
follows:
pannekake:~> gcc-4.6 -m32 -Wall -O2 -march=pentium3 -c fxch.c
The odd sequence is around this:
41: d9 44 24 48 flds 0x48(%esp)
45: dd 5c 24 08 fstpl 0x8(%esp)
49: dd 14 24 fstl (%esp)
4c: d9 5c 24 10 fstps 0x10(%esp)
50: e8 fc ff ff ff call 51 <process+0x51>
55: d9 5c 24 1c fstps 0x1c(%esp)
59: d9 44 24 1c flds 0x1c(%esp)
5d: d9 44 24 10 flds 0x10(%esp)
61: d9 c9 fxch %st(1)
63: d9 1c b7 fstps (%edi,%esi,4)
66: 46 inc %esi
67: 39 ee cmp %ebp,%esi
In particular, why did it use fstps immediately followed by flds of the same
value? And if it really wants to reload (in my more complex example, it really
needs to), why not just do the loads in the right order from the start instead
of doing the fxch?
