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Bug 1677652 - ppc64le: Double-to-float conversion uses wrong rounding mode when followed by memcpy
Summary: ppc64le: Double-to-float conversion uses wrong rounding mode when followed by...
Keywords:
Status: CLOSED ERRATA
Alias: None
Product: Red Hat Enterprise Linux 8
Classification: Red Hat
Component: gcc
Version: 8.1
Hardware: Unspecified
OS: Unspecified
unspecified
unspecified
Target Milestone: rc
: 8.0
Assignee: Marek Polacek
QA Contact: Alexandra Petlanová Hájková
URL:
Whiteboard:
Depends On: 1680182 1682592
Blocks: 1639502 1684553 1684559
TreeView+ depends on / blocked
 
Reported: 2019-02-15 13:45 UTC by Victor Stinner
Modified: 2023-07-18 14:19 UTC (History)
8 users (show)

Fixed In Version: gcc-8.3.1-4.el8
Doc Type: No Doc Update
Doc Text:
Clone Of:
Environment:
Last Closed: 2019-11-05 22:16:35 UTC
Type: Bug
Target Upstream Version:
Embargoed:

Attachments (Terms of Use)


Links
System ID Private Priority Status Summary Last Updated
GNU Compiler Collection 88892 0 P2 RESOLVED [8 Regression] Double-to-float conversion uses wrong rounding mode when followed by memcpy 2020-01-06 09:22:46 UTC
Red Hat Product Errata RHBA-2019:3565 0 None None None 2019-11-05 22:16:57 UTC

Description Victor Stinner 2019-02-15 13:45:18 UTC 
Would it be possible to get the following PPC64LE fix in GCC 8.2 of RHEL 8.1?

The bug has been fixed in master and 8.3 branches of GCC.

Copy of initial message of:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88892

---
With gcc-8.2.1-6.fc28.ppc64le, this code

void
f (double d, char *target)
{
  float f = d;
  __builtin_memcpy (target, &f, sizeof (f));
}

results (with -O2) in (omitting the unwinding information):

f:
        stfs 1,0(4)
        blr

This is incorrect because stfs rounds to zero.  An frsp instruction is missing before the stfs (and would be emitted without the memcpy).

(GCC documents that the default rounding mode “is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations”; see -frounding-mode.)

Found by the Python testsuite: https://bugs.python.org/issue35752
---

Links:

* GCC upstream bug (fixed!): https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88892
* Python upstream bug: https://bugs.python.org/issue35752
* Python downstream bug reported in Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=1540995
Comment 1 Victor Stinner 2019-03-26 14:09:49 UTC 
Any update on this bug? We need the fix for Python: bz#1639502.
Comment 2 Marek Polacek 2019-04-09 16:53:55 UTC 
The upstream fix has been committed to 8.3, so the fix will be included in the next update from upstream.  I'm not sure when exactly that will happen.
Comment 3 Miro Hrončok 2019-05-07 13:11:46 UTC 
Is this planned to be backported to RHEL 8.1.0?
Comment 4 Marek Polacek 2019-05-07 14:08:18 UTC 
(In reply to Miro Hrončok from comment #3)
> Is this planned to be backported to RHEL 8.1.0?

Yes.  In fact, I'm about to start the build; it should be available today.
Comment 5 Miro Hrončok 2019-05-07 14:15:32 UTC 
Excellent! Thank You.
Comment 9 Alexandra Petlanová Hájková 2019-08-05 20:40:56 UTC 
Hello Marek,

there might be some problem,

void
f (double d, char *target)
{
  float f = d;
  __builtin_memcpy (target, &f, sizeof (f));
}

still results (with -O2) in

        stfs 1,0(4)
        blr

for both gcc-8.3.1-4.4.el8.ppc64le and gcc-8.3.1-4.5.el8.ppc64le on power9.
Comment 10 Marek Polacek 2019-08-07 23:53:00 UTC 
I think this is fine.  The original bug report says "An frsp instruction is missing before the stfs."
If I use gcc-8.2.1-3.5 (which doesn't have the fix), then compiling the testcase results in:

f:
.LFB0:
	.cfi_startproc
	stfs 1,0(4)
	blr

whereas if I use gcc-8.3.1-4.5 (which has the fix):

f:
.LFB0:
	.cfi_startproc
	frsp 1,1
	stfs 1,0(4)
	blr

which has "frsp" before "stfs".
Comment 13 errata-xmlrpc 2019-11-05 22:16:35 UTC 
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:3565
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