On Thu, 2010-04-29 at 10:51 +1000, Anton Blanchard wrote: > Hi, > # gcc -pg -mprofile-kernel > > 0000000000000000 <.foo>: > 0: 7c 08 02 a6 mflr r0 > 4: f8 01 00 10 std r0,16(r1) > 8: 48 00 00 01 bl 8 <.foo+0x8> <--- call to mcount > > c: 7c 08 02 a6 mflr r0
Why the extra mflr? Can't we just make it a requirement that mcount returns with r0 back to what it was? -- Steve > 10: f8 01 00 10 std r0,16(r1) > 14: f8 21 ff d1 stdu r1,-48(r1) > 18: e9 22 00 00 ld r9,0(r2) > 1c: e8 69 00 02 lwa r3,0(r9) > 20: 38 21 00 30 addi r1,r1,48 > 24: e8 01 00 10 ld r0,16(r1) > 28: 7c 08 03 a6 mtlr r0 > 2c: 4e 80 00 20 blr > > > This mean we could support ftrace function trace with very little overhead. > > In fact if we are careful when switching to the new mcount ABI and don't > rely on the store of r0, we could probably optimise this even further in a > future gcc and remove the store completely. mcount would be 2 instructions: > > mflr r0 > bl 8 <.foo+0x8> > > Anton _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
