https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52628
--- Comment #2 from Oleg Endo <olegendo at gcc dot gnu.org> --- To catch cases such as int test_01 (int a, int b, int c) { return c << (a > b ? 1 : 0); } a shift with treg_set_expr can be implemented. Combine is looking for this pattern: Failed to match this instruction: (set (reg:SI 169) (ashift:SI (reg:SI 6 r6 [ c ]) (gt:SI (reg:SI 4 r4 [ a ]) (reg:SI 5 r5 [ b ])))) However, this will only be tried with dynamic shifts. If software dynamic shifts are used the library call is expanded too early and combine does not try it. This is done to get constant sharing for the library address. It'd be better to have dynamic shifts only throughout combine, expand library calls in split1 and then do a constant optimization afterwards. For cases such as int test_01 (int a, int b, int c) { return c << (a > b ? 3 : 2); } with dynamic shifts we currently get: cmp/gt r5,r4 mov r6,r0 movt r1 add #2,r1 rts shld r1,r0 where the expected code would be: cmp/gt r5,r4 shll2 r6 bf 0f add r6,r6 0: rts mov r6,r0 or cmp/gt r5,r4 mov #1,r2 mov r6,r0 addc r2,r2 rts shld r2,r6 It fails to use the addc insn because of PR 65317 and PR 67057. Then, the actual shift is tried as: Failed to match this instruction: (set (reg:SI 168) (ashift:SI (reg:SI 6 r6 [ c ]) (plus:SI (reg:SI 169) (const_int 2 [0x2])))) and as: Failed to match this instruction: (set (reg:SI 168) (ashift:SI (reg:SI 6 r6 [ c ]) (plus:SI (gt:SI (reg:SI 4 r4 [ a ]) (reg:SI 5 r5 [ b ])) (const_int 2 [0x2])))) these need to be implemented to be able to split out the common constant shift count and the dynamic 0/1 shift count. For int test_02 (int a, int b, int c) { return c << (a > b ? 2 : 0); } combine tries: Failed to match this instruction: (set (reg:SI 168) (ashift:SI (reg:SI 6 r6 [ c ]) (ashift:SI (gt:SI (reg:SI 4 r4 [ a ]) (reg:SI 5 r5 [ b ])) (const_int 1 [0x1])))) However, for int test_02 (int a, int b, int c) { return c << (a > b ? 3 : 0); } it doesn't try anything like that. This is probably a missed case in ifcvt.