[llvm-dev] Possible to query type information from a malloc in optimized codes

[David Blaikie via llvm-dev]

Type information isn't preserved in LLVM IR-  the debug info will provide a
best-effort, but optimizations might pull apart structures, collapse values
across different variables, etc. So it's potentially lossy.

You can either use debug info (which carries as much type information as is
available right now - many quality of implementation issues/areas of
improvement where the debug information is lossy) or potentially insert
your own intrinsics in the frontend to track the properties you care about.

- Dave

On Thu, Jul 19, 2018 at 10:59 AM Hück, Alexander via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
>
> I am working on a pass that tries to extract type information from, say,
> all malloc statements in LLVM-IR (source language is C).
>
> For debug code, this can be achieved by looking up the respective bitcast
> instruction and extracting the type from it.
>
>
> However, in optimized code, the LLVM-IR omits these direct bitcasts in
> different scenarios (see example after the question).
>
>
> My question now, is there any way to use, e.g., debug data or some use-def
> search to reliably extract the correct type information for such a malloc?
>
>
>
> For one instance, consider the following C code:
>
>   typedef struct {
>     int nvars;
>     int* vars;
>   } struct_grid;
>
>   void set(struct_grid* pgrid, int nvars, int* vars_n) {
>     int* new_vars;
>     new_vars = (int*)malloc(nvars * sizeof(int));
>     for (int i = 0; i < nvars; i++) {
>       new_vars[i] = vars_n[i];
>     }
>     pgrid->vars = new_vars;
>   }
>
> Compiled with -g, we get the expected bitcast. With optimizations, we get:
>
>   %6 = tail call i8* @malloc(i64 %5) ; the malloc, no subsequent bitcast
>
>   ...
>
>   call void @llvm.memcpy.p0i8.p0i8.i64(i8* %6, i8* %10, i64 %12, i32 4, i1
> false)
>
>
> Thus, the %6 is never casted, as it is directly put into the memcpy
> operation.
>
>
> Only later, through some indirection when new_vars is assigned to
> pgrid->vars can we get the real type:
>
>   %14 = getelementptr inbounds %struct.struct_grid, %struct.struct_grid*
> %0, i64 0, i32 1, !dbg !38
>   %15 = bitcast i32** %14 to i8**, !dbg !39
>   store i8* %6, i8** %15, align 8, !dbg !39, !tbaa !40
>   ret void
>
>
>
>
> Thanks in advance.
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>
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