[LLVMbugs] [Bug 4589] Invalid GEP optimization after bitcast

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=4589


Bill Wendling <wendling at apple.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |wendling at apple.com
             Status|NEW                         |RESOLVED
         Resolution|                            |WORKSFORME




--- Comment #1 from Bill Wendling <wendling at apple.com>  2009-07-19 22:05:45 ---
I'm assuming that the original code is this:

$ cat t.c
#include <inttypes.h>

struct dt {
  int32_t f1;
  int32_t *f2;
};

int32_t **get_el(char *arg) {
  struct dt *ptr = (struct dt*)arg;
  return &ptr->f2;
}

On a 32-bit machine, we get:

$ llvm-gcc -o - t.c -S -emit-llvm -Os
; ModuleID = 't.c'
target datalayout =
"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
target triple = "i386-apple-darwin9.8"

define i32** @get_el(i8* %arg) nounwind readnone optsize {
entry:
        %0 = getelementptr i8* %arg, i32 4              ; <i8*> [#uses=1]
        %1 = bitcast i8* %0 to i32**            ; <i32**> [#uses=1]
        ret i32** %1
}

On a 64-bit machine, we get:

$ llvm-gcc -o - t.c -S -emit-llvm -Os -m64
; ModuleID = 't.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
target triple = "x86_64-apple-darwin9.8"

define i32** @get_el(i8* %arg) nounwind readnone optsize {
entry:
        %0 = getelementptr i8* %arg, i64 8              ; <i8*> [#uses=1]
        %1 = bitcast i8* %0 to i32**            ; <i32**> [#uses=1]
        ret i32** %1
}

Both of which are correct.

Note, that LLVM code is not 100% platform independent. Because of specifics of
languages, the LLVM IR that's generated is much different for, say, 32-bit
machines as opposed to 64-bit machines.


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