https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80491
Bug ID: 80491
Summary: Compiler regression for long-add case.
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: khim at google dot com
Target Milestone: ---
Simple long addition program.
struct pair {
uint64_t low;
uint64_t hi;
};
pair add(pair& a, pair& b) {
pair s;
s.low = a.low + b.low;
s.hi = a.hi + b.hi + (s.low < a.low); //carry
return s;
}
Old versions of GCC produced adequate code:
$ gcc -S -O3 test.cc -o-
.file "test.cc"
.text
.p2align 4,,15
.globl _Z3addR4pairS0_
.type _Z3addR4pairS0_, @function
_Z3addR4pairS0_:
.LFB4:
.cfi_startproc
movq (%rdi), %rax
movq 8(%rsi), %rdx
xorl %ecx, %ecx
addq 8(%rdi), %rdx
addq (%rsi), %rax
setc %cl
addq %rcx, %rdx
ret
.cfi_endproc
.LFE4:
.size _Z3addR4pairS0_, .-_Z3addR4pairS0_
.ident "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4"
.section .note.GNU-stack,"",@progbits
New version of gcc produces comparison and jump:
$ gcc -S -O3 test.cc -o-
.file "test.cc"
.text
.p2align 4,,15
.globl _Z3addR4pairS0_
.type _Z3addR4pairS0_, @function
_Z3addR4pairS0_:
.LFB4:
.cfi_startproc
xorl %ecx, %ecx
movq (%rsi), %rax
addq (%rdi), %rax
jc .L5
.L2:
movq 8(%rsi), %rdx
addq 8(%rdi), %rdx
addq %rcx, %rdx
ret
.L5:
movl $1, %ecx
jmp .L2
.cfi_endproc
.LFE4:
.size _Z3addR4pairS0_, .-_Z3addR4pairS0_
.ident "GCC: (GNU) 7.0.1 20170307 (experimental)"
.section .note.GNU-stack,"",@progbits
For comparison - clang produces perfect code here:
$ clang++ -S -O3 test1.cc -o-
.text
.file "test1.cc"
.globl _Z3addR4pairS0_
.p2align 4, 0x90
.type _Z3addR4pairS0_,@function
_Z3addR4pairS0_: # @_Z3addR4pairS0_
.cfi_startproc
# BB#0:
movq (%rsi), %rax
movq 8(%rsi), %rdx
addq 8(%rdi), %rdx
addq (%rdi), %rax
adcq $0, %rdx
retq
.Lfunc_end0:
.size _Z3addR4pairS0_, .Lfunc_end0-_Z3addR4pairS0_
.cfi_endproc
.ident "Android clang version 4.0.285906 (based on LLVM 4.0.285906)"
.section ".note.GNU-stack","",@progbits
