https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99067
Bug ID: 99067
Summary: Missed optimization for induction variable elimination
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: brian.grayson at sifive dot com
Target Milestone: ---
For RISC-V, this code snippet could eliminate the induction variable for the
loop ending condition.
#include <stdint.h>
int16_t a[1000];
int64_t N = 100;
int64_t found_zero() {
for (int i = 0; i <= N; i++) {
if (a[i] == 0) return 1;
}
return 0;
}
gcc -O3 for RISC-V generates:
.L8:
addi a5,a5,2
blt a2,a4,.L4
.L3:
lh a3,0(a5)
addi a4,a4,1 <-- induction variable update that can be eliminated
bne a3,zero,.L8
li a0,1
ret
.L4:
li a0,0
ret
Is there a reason it doesn't do this transform (written at the C level) to do
pointer comparisons:
for (int16_t* p = &a[0]; p <= &a[N]; p++) { ... }
That C code is able to remove the extra add instruction:
.L15:
bgtu a5,a3,.L12
.L11:
lh a4,0(a5)
addi a5,a5,2
bne a4,zero,.L15
li a0,1
ret
.L12:
li a0,0
ret
I verified the same issue occurs in PowerPC and ARM code-gen, so this isn't
target-specific.
