https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121590
Bug ID: 121590
Summary: Code should be unreachable after exhausting all four
float compare relations
Product: gcc
Version: 15.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: Explorer09 at gmail dot com
Target Milestone: ---
With IEEE 754 floating point, there are only four possible relations for
comparing two floating point numbers. After all four possible relations are
exhausted in conditionals, GCC should recognize the remaining code is
unreachable.
Example code:
```c
#include <math.h>
#include <stdlib.h>
int float_compare(double a, double b) {
if (isless(a, b))
return 1;
if (isgreater(a, b))
return 3;
if (isunordered(a, b))
return 4;
if (a == b)
return 2;
// Should never reach here
abort();
}
```
As long as optimization is enabled (-O1 or above), the call to abort() function
should be optimized out.
I can reproduce this issue in at least x86-64, ARM64 and RV64 (in Compiler
Explorer (godbolt.org)).
For example and reference, ARM64 GCC 15.1 with '-O2 -Wno-float-equal' option
produces:
```assembly
float_compare:
fcmp d0, d1
bpl .L9
mov w0, 1
ret
.L9:
ble .L10
mov w0, 3
ret
.L10:
bvs .L7
mov w0, 2
bne .L17
ret
.L7:
mov w0, 4
ret
.L17:
stp x29, x30, [sp, -16]!
mov x29, sp
bl abort
```
Note the unneeded abort() function call.
