https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95685
Bug ID: 95685 Summary: Loop invariants can't be moved out of the loop Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: bina2374 at gmail dot com CC: kito at gcc dot gnu.org, wilson at gcc dot gnu.org Target Milestone: --- Target: riscv32-unknown-elf Command line: bin/riscv64-unknown-elf-gcc -march=rv32imafc -mabi=ilp32f -O2 -funroll-loops bar.c -S ========== C Source ========== unsigned short bar(unsigned char data, unsigned short crc) { unsigned char i = 0; for (i = 0; i < 3; i++) { data >>= 1; if ((data & 1) == 1) crc ^= 0x2001; } return crc; } ========= GCC asm ========= bar: srli a4,a0,1 andi t0,a4,1 mv a5,a0 mv a0,a1 beq t0,zero,.L2 li t1,8192 # addi t2,t1,1 # 0x2001 xor a0,a1,t2 .L2: srli a1,a5,2 andi a2,a1,1 beq a2,zero,.L3 li a3,8192 # addi a6,a3,1 # 0x2001 xor a0,a0,a6 .L3: srli a7,a5,3 andi t3,a7,1 beq t3,zero,.L4 li t4,8192 # addi t5,t4,1 # 0x2001 xor a0,a0,t5 .L4: ret If I compile it without -funroll-loops, loop invariant code motion works: bar: li a2,8192 # mv a4,a0 li a5,3 mv a0,a1 addi a2,a2,1 # 0x2001 .L3: srli a4,a4,1 addi a5,a5,-1 andi a3,a4,1 andi a5,a5,0xff beq a3,zero,.L2 xor a0,a0,a2 .L2: bne a5,zero,.L3 ret I guess the problem is the order of optimization passes. Because cunroll pass makes the loop no longer a loop, loop2_invariant can't work on it. ======================= Ideal Code Generation ======================= bar: srli a4,a0,1 andi t0,a4,1 mv a5,a0 mv a0,a1 li t1,8192 # addi t2,t1,1 # t2 = 0x2001 beq t0,zero,.L2 xor a0,a1,t2 .L2: srli a1,a5,2 andi a2,a1,1 beq a2,zero,.L3 xor a0,a0,t2 # Use t2 .L3: srli a7,a5,3 andi t3,a7,1 beq t3,zero,.L4 xor a0,a0,t2 # Use t2 .L4: ret