https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44281
--- Comment #19 from Jeffrey A. Law <law at redhat dot com> ---
AFAICT the issue with push_flag_into_global_reg_var is poor register
allocation, perhaps made worse by the x86 backend's constrains on the ashldi3_1
insn.
Loop 0 (parent -1, header bb2, depth 0)
bbs: 2
all: 0r96 1r94 2r93 3r92
modified regnos: 92 93 94 96
border:
Pressure: GENERAL_REGS=2
Hard reg set forest:
0:( 0-2 4-6 8-15 21-52)@0
1:( 0-2 4-6 21-52)@0
2:( 0-2 4-6 37-44)@28000
3:( 0-2 5 6 37-44)@20000
Allocno a0r96 of ALL_REGS(46) has 38 avail. regs 0-2 4-6 21-52, node:
0-2 4-6 21-52 (confl regs = 3 7 16-20 53-79)
Allocno a1r94 of GENERAL_REGS(14) has 14 avail. regs 0-2 4-6 37-44,
node: 0-2 4-6 37-44 (confl regs = 3 7-36 45-79)
Allocno a2r93 of GENERAL_REGS(14) has 14 avail. regs 0-2 4-6 37-44,
node: 0-2 4-6 37-44 (confl regs = 3 7-36 45-79)
Allocno a3r92 of GENERAL_REGS(14) has 13 avail. regs 0-2 5 6 37-44,
node: 0-2 5 6 37-44 (confl regs = 3 4 7-36 45-79)
Pushing a3(r92,l0)(cost 0)
Pushing a2(r93,l0)(cost 0)
Pushing a1(r94,l0)(cost 0)
Pushing a0(r96,l0)(cost 0)
Popping a0(r96,l0) -- assign reg 0
Popping a1(r94,l0) -- assign reg 1
Popping a2(r93,l0) -- assign reg 4
Popping a3(r92,l0) -- assign reg 5
Disposition:
3:r92 l0 5 2:r93 l0 4 1:r94 l0 1 0:r96 l0 0
In particular note a0(r96) going into reg0. At that point, we've lost. We'd
really like to see it go into %ebx, which is a global register variable. The
key insns are:
(insn 11 10 12 2 (parallel [
(set (reg:DI 96 [ D.1874 ])
(ashift:DI (reg/v:DI 3 bx [ global_flag_stack ])
(const_int 8 [0x8])))
(clobber (reg:CC 17 flags))
]) j.c:61 511 {*ashldi3_1}
(expr_list:REG_DEAD (reg/v:DI 3 bx [ global_flag_stack ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 12 11 0 2 (parallel [
(set (reg/v:DI 3 bx [ global_flag_stack ])
(ior:DI (reg:DI 94 [ D.1873 ])
(reg:DI 96 [ D.1874 ])))
(clobber (reg:CC 17 flags))
]) j.c:61 400 {*iordi_1}
(expr_list:REG_DEAD (reg:DI 96 [ D.1874 ])
(expr_list:REG_DEAD (reg:DI 94 [ D.1873 ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil)))))
