https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69615
Bug ID: 69615
Summary: 0 to limit signed range checks don't always use
unsigned compare
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: peter at cordes dot ca
Target Milestone: ---
gcc sometimes misses the unsigned-compare trick for checking if a signed value
is between 0 and limit (where limit is known to be <= INT_MAX).
It seems that gcc fails when the upper limit is a variable, even if I shift or
mask it down to a small range. clang handles this case, so I'm sure I
constructed my test case in a way that could be optimized.
All the code in this bug report is on godbolt, for ease of trying with older
versions of gcc (including for ARM64/ARM/PPC), and with clang / icc13.
http://goo.gl/V7PFmv. (I used -x c to compile as C, even though it only
provides c++ compilers).
This appears to be arch-independent (unless my quick skim of asm for ISAs I
barely know misled me...)
--------
The simplest case is when the upper limit is a compile-time constant. There's
one case where gcc and clang fail to optimize: x<=(INT_MAX-1), or
equivalently, x<INT_MAX.
#include <limits.h>
#include <stdint.h>
extern void ext(void);
// clang and gcc both optimize range checks up to INT_MAX-2 to a single
unsigned compare
void r0_to_imax_2(int x){ if (x>=0 && x<=(INT_MAX-2)) ext(); } // good code
void r0_to_imax_1(int x){ if (x>=0 && x<=(INT_MAX-1)) ext(); } // bad code
void r0_to_imax (int x){ if (x>=0 && x<=(INT_MAX-0)) ext(); } // good code
(test/js. not shown)
gcc 5.3.0 -Ofast -mtune=haswell compiles this to:
r0_to_imax_2:
cmpl $2147483645, %edi # that's 0x7ffffffd
jbe .L4
ret
.L4: jmp ext
r0_to_imax_1:
movl %edi, %eax
subl $0, %eax ## Without any -mtune, uses test edi,edi
js .L5
cmpl $2147483647, %edi # that's 0x7fffffff
je .L5
jmp ext
.L5: ret
ICC13 compiles this last one to cmp edi, 0x7ffffffe / ja, so unless my mental
logic is wrong *and* icc13 is buggy, gcc and clang should still be able to use
the same optimization as for smaller upper-limits. They don't: both clang and
gcc use two compare-and-branches for r0_to_imax_1.
BTW, the movl %edi, %eax / subl $0, %eax sequence is used instead of the test
instruction with -mtune=haswell, and even worse with -march=bdver2 where it
even prevents fusion into a compare-and-branch m-op. I'll file a separate bug
report for that if anyone wants me to. Agner Fog's microarch guide doesn't
mention anything that would give that sequence an advantage over test, unless
I'm missing something. It slows AMD down more than (recent) Intel, but that's
not what tuning for Haswell means. :P
Now, on to the case where the limit is variable, but can easily be proven to
itself be in the range [0 .. INT_MAX-1) or much smaller. (If the limit can be
negative (or unsigned greater than INT_MAX) the optimization is impossible:
INT_MIN and other negative numbers could be "below" the limit.)
// gcc always fails to optimize this to an unsigned compare, but clang succeeds
void rangecheck_var(int64_t x, int64_t lim2) {
//lim2 >>= 60;
lim2 &= 0xf; // let the compiler figure out the limited range of limit
if (x>=0 && x<lim2) ext();
}
compiles to (with -O3, and -mtune=core2 to avoid the sub $0 sillyness):
rangecheck_var:
andl $15, %esi
cmpq %rdi, %rsi
jle .L16
testq %rdi, %rdi
js .L16
jmp ext
.L16: ret
