Bug 110564

Summary:	Incorrect results from floating point computations on x86 when optimized
Product:	gcc	Reporter:	D. Richard Hipp <drh>
Component:	target	Assignee:	Not yet assigned to anyone <unassigned>
Status:	RESOLVED DUPLICATE
Severity:	normal	CC:	dkm, ebotcazou
Priority:	P3
Version:	9.4.0
Target Milestone:	---
Host:		Target:
Build:		Known to work:
Known to fail:		Last reconfirmed:
Attachments:	bug.c - compile and run to demonstrate the problem

Description D. Richard Hipp 2023-07-05 19:17:57 UTC

Created attachment 55484 [details]
bug.c - compile and run to demonstrate the problem

Simplified test case attached.  Incorrect floating point result on Linux x86 when compiling and running as follows:

    gcc bug.c -m32 -O1 && ./a.out

The problem goes away under any one or more of the following conditions:

  *  Omit the -m32 flag (to generate x86_64 code)
  *  Omit the -O flag or use -O0
  *  Add the -ffloat-store option
  *  Uncomment both of the "printf()" statements in the test case

Please advise if I am doing something wrong (undefined or unspecified behavior) so that I might fix it.

SQLite bug report at https://sqlite.org/forum/forumpost/ee7278611394034c.  A work-around has been checked into SQLite at https://sqlite.org/src/info/5d9e9364808793d6.  Please advise if there is a better work-around.

Comment 1 Drea Pinski 2023-07-05 19:22:47 UTC

Either use -fexcess-precision=standard or -std=c99 .

Basically this is a dup of bug 323.

*** This bug has been marked as a duplicate of bug 323 ***

Comment 2 Drea Pinski 2023-07-05 19:25:08 UTC

>undefined or unspecified behavior

Neither. Basically GCC uses excess-precision for the fpu as supplied by x87 (80bit IEEE FPU).  Anyways read bug 323 and all of the specifications around FP math and more.

Comment 3 Eric Botcazou 2023-07-05 23:05:08 UTC

Yes, everybody should use -mfpmath=sse these days on x86.