[LTP] [PATCH] Avoid compiler optimisation of malloc calls

[LTP] [PATCH] Avoid compiler optimisation of malloc calls

[Chris Dearman]

clang understands the malloc library call and can optimise calls to
it. In particular it will avoid calling malloc completely if it
detects that the result is not used.

Signed-off-by: Chris Dearman <chris@mips.com>
---
  .../conformance/interfaces/pthread_cond_init/4-1.c |   19 
++++++++++++-------
  1 files changed, 12 insertions(+), 7 deletions(-)

diff --git 
a/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c 
b/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c
index 287fda1..7932ec2 100644
--- 
a/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c
+++ 
b/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c
@@ -50,7 +50,7 @@
  #define ERR_MSG(f, rc) printf("Failed: func: %s rc: %s (%u)\n", \
  			f, strerror(rc), rc);
  -/* Max memory for child is 81B */
+/* Max memory for child is 1MB */
  #define MAX_MEM	((1<<20))
   /*
@@ -59,8 +59,8 @@
   */
  static void child(void)
  {
-	char *curr;
-	char *prev = NULL;
+	void *curr;
+	void *prev = NULL;
  	struct rlimit rl;
  	pthread_cond_t cond;
  	pthread_condattr_t attr;
@@ -76,10 +76,15 @@ static void child(void)
  		exit(PTS_UNRESOLVED);
  	}
  -	/* Consume all memory we can */
-	do {
-		curr = malloc(1);
-	} while (curr);
+	/*
+	 * Consume all memory we can
+	 * It's important to use the malloc() return value in a
+	 * meaningful way to bypass potential compiler optimisations.
+	 */
+	while ((curr = malloc(sizeof(void *)))) {
+		*(void **)curr = prev;
+		prev = curr;
+	}
  	if (errno != ENOMEM) {
  		ERR_MSG("malloc()", errno);
  		exit(PTS_UNRESOLVED);
-- 
1.7.1


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Re: [LTP] [PATCH] Avoid compiler optimisation of malloc calls

[chrubis]

Hi!
> clang understands the malloc library call and can optimise calls to
> it. In particular it will avoid calling malloc completely if it
> detects that the result is not used.

Both patches are okay, but seems to be corrupted (git am wasn't able to
understand them).

Could you send these again (possibly with git send-email) or send a pull
request from repo forked on github or anything else that would work?

-- 
Cyril Hrubis
chrubis@suse.cz

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[LTP] [PATCH] Avoid compiler optimisation of malloc calls

[Chris Dearman]

[-- Attachment #1: Type: text/plain, Size: 349 bytes --]

clang understands the malloc library call and can optimise calls to
it. In particular it will avoid calling malloc completely if it
detects that the result is not used.

Signed-off-by: Chris Dearman <chris@mips.com>
---
 .../conformance/interfaces/pthread_cond_init/4-1.c |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

[-- Attachment #2: avoid-compiler-optimisation-of.patch --]
[-- Type: text/plain, Size: 1266 bytes --]

diff --git a/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c b/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c
index 287fda1..06accd6 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c
+++ b/testcases/open_posix_testsuite/conformance/interfaces/pthread_cond_init/4-1.c
@@ -50,7 +50,7 @@
 #define ERR_MSG(f, rc) printf("Failed: func: %s rc: %s (%u)\n", \
 			f, strerror(rc), rc);
 
-/* Max memory for child is 81B */
+/* Max memory for child is 1MB */
 #define MAX_MEM	((1<<20))
 
 /*
@@ -59,8 +59,8 @@
  */
 static void child(void)
 {
-	char *curr;
-	char *prev = NULL;
+	void *curr;
+	void *prev = NULL;
 	struct rlimit rl;
 	pthread_cond_t cond;
 	pthread_condattr_t attr;
@@ -76,10 +76,15 @@ static void child(void)
 		exit(PTS_UNRESOLVED);
 	}
 
-	/* Consume all memory we can */
-	do {
-		curr = malloc(1);
-	} while (curr);
+	/*
+	 * Consume all memory we can
+	 * It's importamt to use the malloc() return value in a
+	 * meaningful way to bypass potential compiler optimisations.
+	 */
+	while ((curr = malloc(sizeof(void *)))) {
+		*(void **)curr = prev;
+		prev = curr;
+	}
 	if (errno != ENOMEM) {
 		ERR_MSG("malloc()", errno);
 		exit(PTS_UNRESOLVED);

[-- Attachment #3: Type: text/plain, Size: 240 bytes --]

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Re: [LTP] [PATCH] Avoid compiler optimisation of malloc calls

[Wanlong Gao]

On 10/18/2012 01:35 AM, Chris Dearman wrote:
> clang understands the malloc library call and can optimise calls to
> it. In particular it will avoid calling malloc completely if it
> detects that the result is not used.
> 
> Signed-off-by: Chris Dearman <chris@mips.com>

Pushed with Cyril's Ack, thank you.

Wanlong Gao

> ---
>  .../conformance/interfaces/pthread_cond_init/4-1.c |   19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> 
> 
> ------------------------------------------------------------------------------
> Everyone hates slow websites. So do we.
> Make your web apps faster with AppDynamics
> Download AppDynamics Lite for free today:
> http://p.sf.net/sfu/appdyn_sfd2d_oct
> 
> 
> 
> _______________________________________________
> Ltp-list mailing list
> Ltp-list@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/ltp-list
> 


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