https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356
Bug ID: 94356
Summary: Missed optimisation: useless multiplication generated
for pointer comparison
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pascal_cuoq at hotmail dot com
Target Milestone: ---
The closest existing ticket I found for this one is
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48316 but this seems different
enough, although it might be linked.
Consider the function:
typedef int t[100000];
int f(t *p, long long o) {
return p < p+o;
}
GCC 9.3 with -O2, targeting x86-64, correctly simplifies by p, but still
generates a 64x64->64 multiplication in the computation of the offset:
f:
imulq $400000, %rsi, %rsi
xorl %eax, %eax
testq %rsi, %rsi
setg %al
ret
(Compiler Explorer link: https://gcc.godbolt.org/z/_pT8E- )
Clang 10 avoids generating the multiplication on this example:
f: # @f
xorl %eax, %eax
testq %rsi, %rsi
setg %al
retq
A variant of this example is this other function g with two offsets:
int g(t *p, long long o1, long long o2) {
return p+o1 < p+o2;
}
Clang generates the same code as for “o1<o2”,
whereas GCC generates two multiplications:
g:
imulq $400000, %rsi, %rsi
xorl %eax, %eax
imulq $400000, %rdx, %rdx
cmpq %rdx, %rsi
setl %al
ret
Compiler Explorer link: https://gcc.godbolt.org/z/sDJyHP
