https://bugs.llvm.org/show_bug.cgi?id=46066
Bug ID: 46066
Summary: Register allocation does not keep loop-incremented
variables in same register, leading to unnecessary
register-to-register copies
Product: new-bugs
Version: 10.0
Hardware: Other
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: heikki.kult...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
Created attachment 23529
--> https://bugs.llvm.org/attachment.cgi?id=23529&action=edit
C code causing this inefficient code generation
It would seem that the default register allocator is really bad at keeping
variables that are incremented inside loop in the same register, causing lots
of extra register-to-register copies.
C Code:
extern short* A;
extern short* B;
extern short* C;
extern short* end;
int main(void) {
short* a = A;
short* b = B;
short* c = C;
do {
*a++ = *b++ + *c++;
} while (b != end);
}
The assembly of the loop, compiled to riscv32 with -O3 becomes:
lh a4, 0(a2)
lh a5, 0(a3)
addi a1, a2, 2 # this stupidly changes register of the value
addi a3, a3, 2
add a2, a5, a4
addi a4, a0, 2 # this stupidly changes register of the value
sh a2, 0(a0)
add a0, zero, a4 # unnecessary register-to-register move
add a2, zero, a1 # unnecessary register-to-register move
bne a6, a1, .LBB0_1
If the pointers would always stay in the same registers(a0, a2, a3), the code
would be 2 instructions shorter:
lh a4, 0(a2)
lh a5, 0(a3)
addi a2, a2, 2
addi a3, a3, 2
add a1, a5, a4
sh a1, 0(a0)
addi a0, a0, 2 # this gets moved after the store due WaR
bne a6, a2, .LBB0_1
I have encountered the same effect on some other architectures too, but this
case with risc-v was the most clear one, so using this as the example.
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
