https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113023
Bug ID: 113023
Summary: RISCV redundant code for loading fixed address
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: iwfinlay at gmail dot com
Target Milestone: ---
Created attachment 56879
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56879&action=edit
Save temps from: riscv64-unknown-elf-g++ --save-temps -march=rv32imv_zbb_zbs
-mabi=ilp32 -O3 -Wall -Wextra -c test.cpp
Load immediate of a constant leaves a redundant addi x, x, 0 (aka mv x, x) for
the lower bits of the constant in some cases. The generated code is
functionally correct but loses performance.
void pcnt0() updates a global structure. int pcnt1() returns the value. Both
access the global structure for the input but pcnt0() has the redundant mv a5,
a5. Note that the use of cpop is not significant (other functions show the same
behavior in the attached test case).
Disassembly of section .text:
00000028 <_Z5pcnt0v>:
28: 000007b7 lui a5,0x0
2c: 00078793 mv a5,a5
30: 0047a703 lw a4,4(a5) # 4 <_Z6globalv+0x4>
34: 60271713 cpop a4,a4
38: 00e7a023 sw a4,0(a5)
3c: 00008067 ret
00000040 <_Z5pcnt1v>:
40: 000007b7 lui a5,0x0
44: 0047a503 lw a0,4(a5) # 4 <_Z6globalv+0x4>
48: 60251513 cpop a0,a0
4c: 00008067 ret
riscv64-unknown-elf-g++ --save-temps -march=rv32imv_zbb_zbs -mabi=ilp32 -O3
-Wall -Wextra -c test.cpp
riscv64-unknown-elf-objdump -d test.o > test.asm (subset shown above)
A brew install of GCC for RISCV is used for this case. The problem originated
in client code with a customized compiler build (also 12.2).
