From: "agentzh at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: systemtap@sourceware.org
Subject: [Bug translator/26392] New: The optimizer would generate inefficient code which always evaluate dwarf ops eagerly
Date: Fri, 14 Aug 2020 19:58:52 +0000 [thread overview]
Message-ID: <bug-26392-6586@http.sourceware.org/bugzilla/> (raw)
https://sourceware.org/bugzilla/show_bug.cgi?id=26392
Bug ID: 26392
Summary: The optimizer would generate inefficient code which
always evaluate dwarf ops eagerly
Product: systemtap
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: translator
Assignee: systemtap at sourceware dot org
Reporter: agentzh at gmail dot com
Target Milestone: ---
Created attachment 12773
--> https://sourceware.org/bugzilla/attachment.cgi?id=12773&action=edit
bad case using the default optimized translator and good case without
optimization
Consider the following example:
```
stap -e 'global a; probe process.begin { if (a == 1) next; println("===== aa")
println(ubacktrace()) }'
```
Regardless the value of the global variable `a`, the generated C code always
evaluates the value of `ubacktrace()` eagerly even before the `if (a == 1)`
check:
```C
(void)
({
({
c->locals[c->nesting+1].function___global_ubacktrace__overload_0.__retvalue =
&l->__tmp0[0];
c->locals[c->nesting+1].function___global_ubacktrace__overload_0.__cur_frame =
"\011<input>:1:79 in probe process.begin\012";
function___global_ubacktrace__overload_0 (c);
if (unlikely(c->last_error || c->aborted)) goto out;
(void) 0;
});
strlcpy (l->l___stable___global_ubacktrace__overload_0_value, l->__tmp0,
MAXSTRINGLEN);
l->__tmp0;
});
if ((((global(s___global_a)) == (((int64_t)1LL))))) {
goto out;
}
```
Disabling the optimizations make this issue go away:
```
if ((((global(s___global_a)) == (((int64_t)1LL))))) {
c->actionremaining -= 1;
if (unlikely (c->actionremaining <= 0)) {
c->last_error = "MAXACTION exceeded";
c->last_stmt = "keyword at <input>:1:45";
goto out;
}
goto out;
}
...
(void)
({
strlcpy (l->__tmp2,
({
function___global_ubacktrace__overload_0 (c);
if (unlikely(c->last_error || c->aborted)) goto out;
strlcpy (l->__tmp3,
c->locals[c->nesting+1].function___global_ubacktrace__overload_0.__retvalue,
MAXSTRINGLEN);
l->__tmp3;
}), MAXSTRINGLEN);
#ifndef STP_LEGACY_PRINT
c->printf_locals.stp_printf_1.arg0 = l->__tmp2;
stp_printf_1 (c);
#else // STP_LEGACY_PRINT
_stp_printf ("%s\n", l->__tmp2);
#endif // STP_LEGACY_PRINT
if (unlikely(c->last_error || c->aborted)) goto out;
((int64_t)0LL);
});
```
Seems like an optimization doing code moving is just too aggressive.
I've tested the latest git master version on my side (commit f70d74ef66952).
I've attached the two cases' C files (by -p3) for the reference.
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next reply other threads:[~2020-08-14 19:58 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-14 19:58 agentzh at gmail dot com [this message]
2020-08-14 20:59 ` [Bug translator/26392] " agentzh at gmail dot com
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