Incorrect small struct passing in 5th argument position

[sgraham]

I found this prints "1 2 3 4 N" where N is something random (not 99).

This error doesn't occur if only 3 ints are passed before the struct, or if 5 are, so I guess it's probably an edge case in push_args(), or maybe in the setup in ND_FUNCALL.

int printf();

typedef struct StRegSized {
    unsigned char r;
} StRegSized;

void func1(int a, int b, int c, int d, StRegSized st) {
  printf("%d %d %d %d %d\n", a, b, c, d, st.r);
}

int main(void) {
  StRegSized x = {99};
  func1(1, 2, 3, 4, x);
}

[sgraham]

Ah, it's actually in assign_lvar_offsets() I think. It might be that

chibicc/codegen.c

Line 1333 in 90d1f7f

if (fp + fp1 + fp2 < FP_MAX && gp + !fp1 + !fp2 < GP_MAX) {

needs to be <= not <. The other checks that are similar are doing post-increments, so they're checking before the registers-to-be-used have been added.

diff --git a/codegen.c b/codegen.c
index da11fd7..1fdff28 100644
--- a/codegen.c
+++ b/codegen.c
@@ -1330,7 +1330,7 @@ static void assign_lvar_offsets(Obj *prog) {
         if (ty->size <= 16) {
           bool fp1 = has_flonum(ty, 0, 8, 0);
           bool fp2 = has_flonum(ty, 8, 16, 8);
-          if (fp + fp1 + fp2 < FP_MAX && gp + !fp1 + !fp2 < GP_MAX) {
+          if (fp + fp1 + fp2 <= FP_MAX && gp + !fp1 + !fp2 <= GP_MAX) {
             fp = fp + fp1 + fp2;
             gp = gp + !fp1 + !fp2;
             continue;

[sgraham]

Oh, but it looks like there's a second problem near this code too! In this case the c argument isn't correctly passed. Hmm.

int printf();

typedef struct StRegSized {
    unsigned long r;
} StRegSized;

void func(int a, StRegSized st, int b, StRegSized st2, int c) {
  printf("%d %d %d %d %d\n", a, st.r, b, st2.r, c);
}

int main(void) {
  StRegSized x = {99};
  StRegSized y = {98};
  func(1, x, 2, y, 3);
}

[sgraham]

I think maybe assign_lvar_offsets also needs a case for structs that are <= 8 bytes, rather than assuming it should always take two registers for a struct pass.

[iphydf]

Another repro with 1 struct param:

#include <stdio.h>

typedef struct Wrapper {
    int wrapped;
} Wrapper;

static void broken_function(Wrapper wrapper, int param2, int param3, int param4, int param5, int param6)
{
    fprintf(stderr, "%x\n", wrapper.wrapped);
    fprintf(stderr, "%x\n", param2);
    fprintf(stderr, "%x\n", param3);
    fprintf(stderr, "%x\n", param4);
    fprintf(stderr, "%x\n", param5);
    fprintf(stderr, "%x\n", param6);  // should say 66, but says 0
}

int main(void)
{
    Wrapper wrapper = {0x11};
    int param5 = 0x55;
    int param6 = 0x66;
    broken_function(wrapper, 0x22, 0x33, 0x44, param5, param6);
}

I guess the workaround is to never pass structs.

[fuhsnn]

@iphydf I've been fixing ABI issues in my fork; could use some extra eyes if you are testing on the codebase.

[iphydf]

@fuhsnn TokTok/c-toxcore#2624 it works perfectly. I'm adding it to our CI to make sure it keeps working.