<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 6, 2017, at 11:19 AM, James Y Knight via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">AFAIK, this cannot actually work on Apple platforms, because its object file format (Mach-O) doesn't use sections to determine the ranges of code/data to keep together, but instead _infers_ boundaries based on the range between global symbols in the symbol table.<div class=""><div class=""><br class=""></div><div class="">So, the symbol pointing to the beginning of @main *necessarily* makes that be a section boundary.</div></div><div class=""><br class=""></div><div class="">I think the best that could be done in LLVM is to not emit the ".subsections_via_symbols" asm directive (effectively disabling dead stripping on that object) if any prefix data exists. Currently it emits that flag unconditionally for MachO.</div></div></div></blockquote><div><br class=""></div><div>Even disassembling dead-code stripping, how can you guarantee that the linker won’t change the order in which atoms are laid out?</div><div><br class=""></div><div>— </div><div>Mehdi</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Mon, Mar 6, 2017 at 4:40 AM, Moritz Angermann via llvm-dev <span dir="ltr" class=""><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<br class="">
<br class="">
I just came across a rather annoying behavior with llvm 3.9. Assuming the following<br class="">
samle code in test.ll:<br class="">
<br class="">
; Lets have some global int x = 4<br class="">
@x = global i32 10, align 4<br class="">
; and two strings "p = %d\n" for the prefix data,<br class="">
; as well as "x = %d\n" to print the (global) x value.<br class="">
@.str = private unnamed_addr constant [8 x i8] c"x = %d\0A\00", align 1<br class="">
@.str2 = private unnamed_addr constant [8 x i8] c"p = %d\0A\00", align 1<br class="">
<br class="">
; declare printf, we'll use this later for printf style debugging.<br class="">
declare i32 @printf(i8*, ...)<br class="">
<br class="">
; define a main function.<br class="">
define i32 @main() prefix i32 123 {<br class="">
; obtain a i32 pointer to the main function.<br class="">
; the prefix data is right before that pointer.<br class="">
%main = bitcast i32 ()* @main to i32*<br class="">
<br class="">
; use the gep, to cmpute the start of the prefix data.<br class="">
%prefix_ptr = getelementptr inbounds i32, i32* %main, i32 -1<br class="">
; and load it.<br class="">
%prefix_val = load i32, i32* %prefix_ptr<br class="">
<br class="">
; print that value.<br class="">
%ret = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str2, i32 0, i32 0), i32 %prefix_val)<br class="">
<br class="">
; similarly let's do the same with the global x.<br class="">
%1 = alloca i32, align 4<br class="">
store i32 0, i32* %1, align 4<br class="">
%2 = load i32, i32* @x, align 4<br class="">
%3 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str, i32 0, i32 0), i32 %2)<br class="">
ret i32 0<br class="">
}<br class="">
<br class="">
gives the following result (expected)<br class="">
<br class="">
$ clang test.ll<br class="">
$ ./a.out<br class="">
p = 123<br class="">
x = 10<br class="">
<br class="">
however, with -dead_strip on macOS, we see the following:<br class="">
<br class="">
$ clang test.ll -dead_strip<br class="">
$ ./a.out<br class="">
p = 0<br class="">
x = 10<br class="">
<br class="">
Thus I believe we are incorrectly stripping prefix data when linking with -dead_strip on macOS.<br class="">
<br class="">
As I do not have a bugzilla account, and hence cannot post this as a proper bug report.<br class="">
<br class="">
Cheers,<br class="">
Moritz<br class="">
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</blockquote></div><br class=""></div>
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