tmsriram created this revision.
tmsriram added reviewers: rnk, davidxl.
tmsriram added a subscriber: cfe-commits.
clang does not pass pie-level and pic-level option values to the code generator
with "-x ir" due to the following code in CompilerInvocation.cpp:
if (DashX == IK_AST || DashX == IK_LLVM_IR) {
....;
} else {
ParseLangArgs(*Res.getLangOpts(), Args, DashX, Res.getTargetOpts(),
Diags);
}
pie-level and pic-level are LangArgs and are not parsed with "-x ir". They
define macros __PIC__ and __PIE___ when set, see
lib/Frontend/InitPreprocessor.cpp
Not setting LangOpts.PIELevel means that Options.PositionIndependentExecutable
is always false even when -fPIE is used. This patch fixes that.
I considered an alternative of making pie-level and pic-level CodeGen Options.
However, since these options are used to set macros __PIE__, __PIC__, etc. this
looked better as LangOpts. Please let me know what you think.
For a broader context, I am working on optimizing accesses to global variables
with -fPIE on x86-64 and this patch is the first step to get there. In
particular, I noticed the following code generated with clang:
hello.cpp:
int a = 0;
int main() {
return a;
}
$ clang -O2 -fPIE hello.cc -S
$ cat hello.s
main: # @main
movq a@GOTPCREL(%rip), %rax
movl (%rax), %eax
retq
Creating a GOT entry for global 'a' is unnecessary as 'a' is defined in
hello.cpp which will be linked into a position independent executable (fPIE).
Hence, the definition of 'a' cannot be overridden and we can remove a load. The
efficient access is this:
main: # @main
movl a(%rip), %eax
retq
I plan to address this in a separate patch.
http://reviews.llvm.org/D18843
Files:
lib/Frontend/CompilerInvocation.cpp
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2142,6 +2142,12 @@
// what the input type is.
if (Args.hasArg(OPT_fobjc_arc))
Res.getLangOpts()->ObjCAutoRefCount = 1;
+ // PIClevel and PIELevel are needed during code generation and this should
be
+ // set regardless of the input type.
+ Res.getLangOpts()->PICLevel = getLastArgIntValue(Args, OPT_pic_level,
+ 0, Diags);
+ Res.getLangOpts()->PIELevel = getLastArgIntValue(Args, OPT_pie_level,
+ 0, Diags);
parseSanitizerKinds("-fsanitize=", Args.getAllArgValues(OPT_fsanitize_EQ),
Diags, Res.getLangOpts()->Sanitize);
} else {
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2142,6 +2142,12 @@
// what the input type is.
if (Args.hasArg(OPT_fobjc_arc))
Res.getLangOpts()->ObjCAutoRefCount = 1;
+ // PIClevel and PIELevel are needed during code generation and this should be
+ // set regardless of the input type.
+ Res.getLangOpts()->PICLevel = getLastArgIntValue(Args, OPT_pic_level,
+ 0, Diags);
+ Res.getLangOpts()->PIELevel = getLastArgIntValue(Args, OPT_pie_level,
+ 0, Diags);
parseSanitizerKinds("-fsanitize=", Args.getAllArgValues(OPT_fsanitize_EQ),
Diags, Res.getLangOpts()->Sanitize);
} else {
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