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* [Bug middle-end/106559] New: Spurious warning format-truncation (regression from 9) @ 2022-08-08 15:10 phdiv at fastmail dot fm 2022-08-09 7:37 ` [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation " rguenth at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: phdiv at fastmail dot fm @ 2022-08-08 15:10 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106559 Bug ID: 106559 Summary: Spurious warning format-truncation (regression from 9) Product: gcc Version: 10.3.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: phdiv at fastmail dot fm Target Milestone: --- Created attachment 53426 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53426&action=edit Reproducer. > cat w.cpp extern char buffer[64]; extern unsigned int number[16]; void f(){ static const char string[16][3]={ "01","02","03","04","05","06","07","08", "09","10","11","12","13","14","15","16"}; for(unsigned int i=0; i<16; ++i) __builtin_snprintf(buffer,sizeof(buffer),"%u (%s): %8x", i,string[i],number[i]); } > g++ -O1 -Wformat-truncation -c w.cpp w.cpp: In function 'void f()': w.cpp:10:61: warning: '__builtin_snprintf' output may be truncated before the last format character [-Wformat-truncation=] 10 | __builtin_snprintf(buffer,sizeof(buffer),"%u (%s): %8x", | ^ w.cpp:10:23: note: '__builtin_snprintf' output between 17 and 65 bytes into a destination of size 64 10 | __builtin_snprintf(buffer,sizeof(buffer),"%u (%s): %8x", | ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 11 | i,string[i],number[i]); | ~~~~~~~~~~~~~~~~~~~~~~ Granted, the documentation of Wformat-truncation mentions possible false positives, but this is a regression in GCC 10 with respect to GCC 9 (and present up to trunk). For some more variations which do or do not exhibit this bug, see https://godbolt.org/z/PvfrchP7E . ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation (regression from 9) 2022-08-08 15:10 [Bug middle-end/106559] New: Spurious warning format-truncation (regression from 9) phdiv at fastmail dot fm @ 2022-08-09 7:37 ` rguenth at gcc dot gnu.org 2022-08-16 16:00 ` msebor at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: rguenth at gcc dot gnu.org @ 2022-08-09 7:37 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106559 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Target Milestone|--- |10.5 Priority|P3 |P2 Ever confirmed|0 |1 Last reconfirmed| |2022-08-09 Summary|Spurious warning |[10/11/12/13 Regression] |format-truncation |Spurious warning |(regression from 9) |-Wformat-truncation | |(regression from 9) Known to fail| |10.4.0, 11.3.0, 12.1.0, | |9.5.0 Known to work| |8.4.0 --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- Confirmed. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation (regression from 9) 2022-08-08 15:10 [Bug middle-end/106559] New: Spurious warning format-truncation (regression from 9) phdiv at fastmail dot fm 2022-08-09 7:37 ` [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation " rguenth at gcc dot gnu.org @ 2022-08-16 16:00 ` msebor at gcc dot gnu.org 2022-12-14 2:28 ` pinskia at gcc dot gnu.org 2023-07-07 10:43 ` [Bug tree-optimization/106559] [11/12/13/14 " rguenth at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: msebor at gcc dot gnu.org @ 2022-08-16 16:00 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106559 Martin Sebor <msebor at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |msebor at gcc dot gnu.org Blocks| |85741 --- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> --- The warning triggers because it considers the size of the whole `string' array passed as an argument to the %s directive. It does that because the analysis is unable to determine which array element the argument points to and it's not "smart" enough to see that all the elements are strings of the same length. The output of the -fdump-tree-strlen option below helps see what's gooing on (the numbers next to each Result: show the minimum, maximum, likely, and unlikely amount of output produced by the directive, with the corresponding running totals in parentheses). The problem can be reduced to a missed optimization opportunity in the test following test case: the condition in each iteration of the loop is false so the loop can be optimized away, but because of the incomplete analysis above it is not. void f (void) { static const char string[16][3]={ "01","02","03","04","05","06","07","08", "09","10","11","12","13","14","15","16"}; for(unsigned int i=0; i<16; ++i) if (__builtin_strlen (string[i]) != 2) __builtin_abort (); } Short of improving the strlen optimization the warning could also be suppressed by considering the cast in the assignment `_2 = (const char[3] *) ivtmp.11_15;' and using the size of the array as the upper bound on the length of the string. (This wouldn't be safe for the optimization.) Until this is fixed in GCC, the warning can be suppressed and the emitted code improved by asserting in each iteration that the length of the string is (at most) two, like so: if (__builtin_strlen (string[i]) != 2) __builtin_unreachable (); pr106559.c:11: __builtin_snprintf: objsize = 64, fmtstr = "%u (%s): %8x" Directive 1 at offset 0: "%u" Result: 1, 2, 2, 2 (1, 2, 2, 2) Directive 2 at offset 2: " (", length = 2 Result: 2, 2, 2, 2 (3, 4, 4, 4) Directive 3 at offset 4: "%s" Result: 0, 47, 47, 9223372036854775807 (3, 51, 51, -9223372036854775805) Directive 4 at offset 6: "): ", length = 5 Result: 5, 5, 5, 5 (8, 56, 56, -9223372036854775800) Directive 5 at offset 11: "%8x" Result: 8, 8, 8, 8 (16, 64, 64, -9223372036854775792) Directive 6 at offset 14: "", length = 1 pr106559.c: In function ‘f’: pr106559.c:11:61: warning: ‘__builtin_snprintf’ output may be truncated before the last format character [-Wformat-truncation=] 11 | __builtin_snprintf(buffer,sizeof(buffer),"%u (%s): %8x", | ^ pr106559.c:11:5: note: ‘__builtin_snprintf’ output between 17 and 65 bytes into a destination of size 64 11 | __builtin_snprintf(buffer,sizeof(buffer),"%u (%s): %8x", | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 12 | i,string[i],number[i]); | ~~~~~~~~~~~~~~~~~~~~~~ void f () { unsigned long ivtmp.11; unsigned long ivtmp.5; unsigned int i; static const char string[16][3] = {"01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12", "13", "14", "15", "16"}; unsigned int _1; const char[3] * _2; unsigned int _18; <bb 2> [local count: 63136016]: ivtmp.11_17 = (unsigned long) &string; <bb 3> [local count: 1010605809]: # ivtmp.5_13 = PHI <ivtmp.5_14(5), 0(2)> # ivtmp.11_15 = PHI <ivtmp.11_16(5), ivtmp.11_17(2)> _18 = (unsigned int) ivtmp.5_13; _1 = MEM[(unsigned int *)&number + ivtmp.5_13 * 4]; _2 = (const char[3] *) ivtmp.11_15; <<< cast not considered __builtin_snprintf (&buffer, 64, "%u (%s): %8x", _18, _2, _1); <<< warning here for _2 Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85741 [Bug 85741] [meta-bug] bogus/missing -Wformat-overflow ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation (regression from 9) 2022-08-08 15:10 [Bug middle-end/106559] New: Spurious warning format-truncation (regression from 9) phdiv at fastmail dot fm 2022-08-09 7:37 ` [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation " rguenth at gcc dot gnu.org 2022-08-16 16:00 ` msebor at gcc dot gnu.org @ 2022-12-14 2:28 ` pinskia at gcc dot gnu.org 2023-07-07 10:43 ` [Bug tree-optimization/106559] [11/12/13/14 " rguenth at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2022-12-14 2:28 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106559 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |cjamcl at gmail dot com --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- *** Bug 108091 has been marked as a duplicate of this bug. *** ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/106559] [11/12/13/14 Regression] Spurious warning -Wformat-truncation (regression from 9) 2022-08-08 15:10 [Bug middle-end/106559] New: Spurious warning format-truncation (regression from 9) phdiv at fastmail dot fm ` (2 preceding siblings ...) 2022-12-14 2:28 ` pinskia at gcc dot gnu.org @ 2023-07-07 10:43 ` rguenth at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: rguenth at gcc dot gnu.org @ 2023-07-07 10:43 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106559 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Target Milestone|10.5 |11.5 --- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> --- GCC 10 branch is being closed. ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-07-07 10:43 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2022-08-08 15:10 [Bug middle-end/106559] New: Spurious warning format-truncation (regression from 9) phdiv at fastmail dot fm 2022-08-09 7:37 ` [Bug tree-optimization/106559] [10/11/12/13 Regression] Spurious warning -Wformat-truncation " rguenth at gcc dot gnu.org 2022-08-16 16:00 ` msebor at gcc dot gnu.org 2022-12-14 2:28 ` pinskia at gcc dot gnu.org 2023-07-07 10:43 ` [Bug tree-optimization/106559] [11/12/13/14 " rguenth at gcc dot gnu.org
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