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From: "michael.hudson at linaro dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/59744] New: miscompilation of unsigned comparison on aarch64 Date: Fri, 10 Jan 2014 00:30:00 -0000 [thread overview] Message-ID: <bug-59744-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59744 Bug ID: 59744 Summary: miscompilation of unsigned comparison on aarch64 Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: michael.hudson at linaro dot org Hi, This slightly strangely written program (it's distilled down from frame_offset_overflow in the gcc source itself) should print "bigger" if the first argument is bigger than 10 (or negative, but let's ignore that please): #include <stdlib.h> #include <stdio.h> int a[2] = { 10, 20 }; int is_bigger (long offset, int index) { unsigned long size = -offset; if (size > a[index]) { printf("bigger\n"); return 1; } return 0; } int main (int argc, char** argv) { long v; v = atol(argv[1]); is_bigger(-v, 0); return 0; } When compiled at -O1 or above (and with inlining disabled at -O2 and above), though, it bungles the 0 case: (t-doko)mwhudson@arm64:~$ gcc-4.9 -O3 test.c -o test -fno-inline -Wall (t-doko)mwhudson@arm64:~$ ./test 1 (t-doko)mwhudson@arm64:~$ ./test 11 bigger (t-doko)mwhudson@arm64:~$ ./test 0 bigger (t-doko)mwhudson@arm64:~$ gcc-4.9 -O0 test.c -o test -Wall (t-doko)mwhudson@arm64:~$ ./test 1 (t-doko)mwhudson@arm64:~$ ./test 11 bigger (t-doko)mwhudson@arm64:~$ ./test 0 (t-doko)mwhudson@arm64:~$ What's going on? Here's the disassembly of is_bigger (at O3): 0000000000400608 <is_bigger>: 400608: b0000082 adrp x2, 411000 <_GLOBAL_OFFSET_TABLE_+0x28> 40060c: 91010042 add x2, x2, #0x40 400610: a9bf7bfd stp x29, x30, [sp,#-16]! 400614: 52800003 mov w3, #0x0 // #0 400618: 910003fd mov x29, sp 40061c: b8a1d841 ldrsw x1, [x2,w1,sxtw #2] 400620: ab00003f cmn x1, x0 400624: 540000a2 b.cs 400638 <is_bigger+0x30> 400628: 90000000 adrp x0, 400000 <_init-0x3f8> 40062c: 911b6000 add x0, x0, #0x6d8 400630: 97ffff90 bl 400470 <puts@plt> 400634: 52800023 mov w3, #0x1 // #1 400638: 2a0303e0 mov w0, w3 40063c: a8c17bfd ldp x29, x30, [sp],#16 400640: d65f03c0 ret Basically it seems that the condition "-offset > val" is being compiled as "val + offset does not overflow", which is not valid for offset == 0.
next reply other threads:[~2014-01-10 0:30 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2014-01-10 0:30 michael.hudson at linaro dot org [this message] 2014-01-10 1:00 ` [Bug target/59744] " pinskia at gcc dot gnu.org 2014-01-10 15:21 ` rearnsha at gcc dot gnu.org 2014-01-10 15:23 ` rearnsha at gcc dot gnu.org 2014-01-13 1:00 ` michael.hudson at linaro dot org 2014-01-13 9:34 ` rearnsha at gcc dot gnu.org 2014-01-13 9:35 ` ktkachov at gcc dot gnu.org 2014-08-11 22:08 ` yroux at gcc dot gnu.org
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