Brittle compilation errors

[Erhannis]

I'm getting some very suspicious compilation errors, where I'll get e.g. "undefined reference to 'sin'" on one line, but it was fine five lines prior. I'm ALSO getting similar errors in file Y, depending on whether or not file X has a particular if/else block commented out. It smells of things going wrong in the compiler, triggered by things moved around by optimization.

See https://github.com/Erhannis/minimodem/tree/feature/nestedvm_weirdness - if you have nestedvm installed, and enter src/, and run nestedvm.sh, I think it should fail to compile. Then if you change 3 == 1+1 to 2 == 1+1 in minimodem.c (I've been doing very eccentric things, trying to find the boundaries of the problem), it should compile.

The other place I've been having trouble is simple-tone-generator.c ; I can use sin etc. all I want in fprintf and so forth, but the calls that matter trigger an undefined reference problem on linking.

[Erhannis]

I've refined it a bit. Here is compilation and output of two cases (copied by hand, because vm):

#include <stdio.h>
#include <math.h>

int main(int argc, char *argv[]) {
    fprintf(stderr, "Blah %f\n", sin(4.0f));
    return 0;
}

yields

Blah -0.756802

Whereas the following apparently equivalent file

#include <stdio.h>
#include <math.h>

int main(int argc, char *argv[]) {
    float f = 4.0f;
    fprintf(stderr, "Blah %f\n", sin(f));
    return 0;
}

yields

test.o: In function `main':
(.text.main+0x5c): undefined reference to `sin'
collect2: error: ld returned 1 exit status

Any idea why one would work and the other would fail? The compilation and linking commands were, respectively:

mips-unknown-elf-gcc -DNESTEDVM -DHAVE_CONFIG_H -I. -I.. -O0 -mmemcpy -ffunction-sections -fdata-sections -falign-functions=512 -fno-rename-registers -fno-schedule-insns -fno-delayed-branch -march=mips1 -specs=/usr/local/nestedvm/mips-unknown-elf/lib/crt0-override.spec -I. -Wall -Wno-unused -c -o test.o test.c

and

mips-unknown-elf-gcc -march=mips1 -specs=/usr/local/nestedvm/mips-unknown-elf/lib/crt0-override.spec --static -Wl,--gc-sections -o test.mips test.o