| Issue |
157797
|
| Summary |
Float to int16_t conversion error.
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tomaso-cerebras
|
**Short summary:** It seems that converting 32768.0f to int16_t yields 32768, and not he expected -32768 with -O1 and -O2.
**Compiler version:**
mac-laptop> g++ --version
Apple clang version 17.0.0 (clang-1700.0.13.5)
Target: arm64-apple-darwin24.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
**Reproducer code and commands**
Assuming the C++ code below is stored in the file intdivbug.cc, incorrect (with -O1 and -O2) and correct behavior (-O0) is shown below. If the flag enable_big is changed and set to false, int16_t result to memory before use (through volatile variable), and results in correct results at all three optimization levels (-O0, -O1, and -O2).
_Correct behavior:_
mac-laptop> g++ -Wall -O0 -o intdivbug intdivbug.cc
mac-laptop> ./intdivbug
Finished.
_Incorrect behavior (with -O1):_
mac-laptop> g++ -Wall -O0 -o intdivbug intdivbug.cc
mac-laptop> ./intdivbug
a = -32768, b = -1, ce = -32768, c = 32768
Finished.
_Incorrect behavior (with -O1):_
mac-laptop> g++ -Wall -O2 -o intdivbug intdivbug.cc
max-laptop> ./intdivbug
a = -32768, b = -1, ce = -32768, c = 32768
Finished.
**Reproducer code**
// Test driver for integer division.
#include <cstdio>
#include <cstdint>
typedef float sp;
//typedef short xp;
typedef int16_t xp;
xp intdiv(xp a,xp b) {
sp
af = a,
bf = b,
cf = (af/bf)*(1+1e-6);
xp c = cf;
return c;
}
const bool enable_bug = true;
volatile xp cv[1];
int main(void) {
xp lim = 32767;
for(int ai = -lim-1; ai <= -lim; ai++)
for(int bi = -lim-1; bi <= lim; bi++)
if(bi != 0) {
xp a = ai,b = bi;
xp ce = a/b,c;
if(enable_bug) {
c = intdiv(a,b);
if(ce != c)
printf("a = %d, b = %d, ce = %d, c = %d\n",a,b,ce,(int) c);
} else {
cv[0] = intdiv(a,b);
if(ce != cv[0])
printf("a = %d, b = %d, ce = %d, c = %d\n",a,b,ce,(int) cv[0]);
}
}
printf("Finished.\n");
return 0;
}
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