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Re: [Tinycc-devel] Fwd: TCC VLA bug?

From: Michael Matz
Subject: Re: [Tinycc-devel] Fwd: TCC VLA bug?
Date: Tue, 7 Dec 2021 16:08:41 +0100 (CET)
User-agent: Alpine 2.21 (LSU 202 2017-01-01)


On Sun, 5 Dec 2021, Herman ten Brugge via Tinycc-devel wrote:

On 12/1/21 15:18, grischka wrote:
      Got this report on private email.  Not sure what it means ...

      Output of the code below if compiled with TCC is pretty messy:
      array values are "misplaced" and overwrite each other.
      But everything's ok if compiled with GCC.

The patch below seems to fix it. Can I push it?


diff --git a/tccgen.c b/tccgen.c
index e0b5fd6..67e205b 100644
--- a/tccgen.c
+++ b/tccgen.c
@@ -3494,7 +3494,7 @@ redo:
            type1 = vtop[-1].type;
-            if (vtop[-1].type.t & VT_VLA)
+            if (vtop[-1].type.ref->type.t & VT_VLA)

Hmm, that would mean the VT_VLA flags are wrongly set. The invariant of vla_runtime_pointed_size(type...) is that type->t has VT_VLA (and that type->ref is meaningful). If your patch helps in this situation that means the flag setting is going wrong somewhere. I see that the testcase has doubly-vla types, maybe only the innermost level is marked, but a variable length type is of course variably length if any of the referred types is, so maybe that's the problem. Just speculation, though.

So, the problematic type was:

  int (*)[a][b]

That's pointer to an vla-array of an vla-array of int. All three (inner array, outer array and pointer, but not the int) should be marked VT_VLA. In TCC we collapse the outer array+pointer into one type (a pointer that also has VT_ARRAY/VT_VLA set), so there actually should be two levels: the inner level a VT_VLA pointing to the VT_INT (with its VLA runtime length being evaluated to sizeof(int) * b) and the outer level a VT_VLA pointing to the inner VT_VLA (and its VLA runtime length being evaluated to inner length * a).

I'm surprised that your patch didn't cause other problems, it seems either the testsuite isn't testing VLAs very much, or that the VT_VLA flag is set on types where it shouldn't have been (e.g. on the VT_INT type that is in the type->ref of the 'int [n]' array type).


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