The bitwise-instruction optimizer doesn't know about alignment of globals.

[llvmbot]

Bugzilla Link	2206
Resolution	FIXED
Resolved on	Apr 19, 2008 17:18
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@sunfishcode

Extended Description

Consider "i64 and (i64 ptrtoint (i64 (i64)* @​cons to i64), i64 -8)"[1].
(Where @​cons is a global function with an alignment >= 8 specified)
Such expressions aren't simplified by "opt -std-compile-opts", even though the relevant bits of the ptrtoint result are guaranteed to be zero because of above-mentioned alignment and similar optimizations are performed if the pointer is replaced by an integer.

[1]: That expression was shortened for readability, and not actually tested in this form. The actual expression was "i64 inttoptr (i64 and (i64 or (i64 ptrtoint (i64 (i64)* @​cons to i64), i64 1), i64 -8) to i64 (i64)*)".

[lattner]

Please include a testcase, preferably in C form. Thanks!

[llvmbot]

I couldn't get llvm-gcc to let me specify alignment on a function, but the same happens for other global data:

#include <stdlib.h>

int cons attribute ((aligned(8)));

unsigned long foo() {
return ((size_t) &cons) & 7;
}

output:

$ llvm-gcc -c llvm-func-and.c -emit-llvm -o out.bc -O3 && llvm-dis -o - out.bc
; ModuleID = 'out.bc'
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-unknown-linux-gnu"
@​cons = weak global i32 0, align 8 ; <i32*> [#uses=1]

define i64 @​foo() nounwind {
entry:
ret i64 and (i64 ptrtoint (i32* @​cons to i64), i64 7)
}

[sunfishcode]

With this:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080407/060935.html
the bitwise-instruction optimizer now knows some things about alignment of globals.

However, the given testcase isn't yet simplified because it uses a constant expression
instead of instructions, so it would need to be simplified by the constant folder.

[lattner]

implemented:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080414/061282.html

[edwintorok]

mentioned in issue llvm/llvm-bugzilla-archive#2240