http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61051
Bug ID: 61051 Summary: Duplicated instructions in both conditional branches Product: gcc Version: 4.10.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: carrot at google dot com Target: powerpc64le-grtev4-linux-gnu Source code: extern int* foo1 ( long* ); extern int *foo2 ( long *, long *); extern void foo3 (long, long); void bar() { long d, f, x, s, r; int *p; d = 1; if (foo1(&r)) { p = foo2( &d, &x); if( x != *p ) s = 1; else s = 2; if( r > 0 ) f = 1 + d; else f = d; foo3 (f, d); *p = s; } } Compile it with options -O2 -m64 -mcpu=power8, gcc generates: bar: 0: addis 2,12,.TOC.-0b@ha addi 2,2,.TOC.-0b@l .localentry bar,.-bar mflr 0 std 30,-16(1) std 31,-8(1) li 9,1 std 0,16(1) stdu 1,-80(1) addi 3,1,32 std 9,48(1) bl foo1 nop cmpdi 7,3,0 beq 7,.L1 addi 3,1,48 addi 4,1,40 bl foo2 nop ld 10,40(1) li 30,1 lwa 9,0(3) mr 31,3 cmpd 7,9,10 beq 7,.L12 // C ld 9,32(1) // A1 cmpdi 7,9,0 // A2 ble 7,.L4 .L13: ld 4,48(1) addi 3,4,1 .L5: bl foo3 nop stw 30,0(31) .L1: addi 1,1,80 ld 0,16(1) ld 30,-16(1) ld 31,-8(1) mtlr 0 blr .p2align 4,,15 .L12: ld 9,32(1) // B1 li 30,2 cmpdi 7,9,0 // B2 bgt 7,.L13 .L4: ld 3,48(1) mr 4,3 b .L5 .long 0 .byte 0,0,0,1,128,2,0,0 .size bar,.-bar Instruction C is a conditional branch. In both branches A1 and B1 are same instructions, they can be moved to before C. If we use a different conditional register, instructions A2 and B2 can also be moved before C.