-Warray-bounds warning with asm goto in impossible switch cases

[nathanchance]

Bugzilla Link	51682
Version	trunk
OS	Windows NT
Blocks	#4440
Attachments	False positive -Warray-bounds reproducer
CC	@DougGregor,@efriedma-quic,@kees,@nickdesaulniers,@pageexec,@zygoloid

Extended Description

A piece of code in the Linux kernel triggers -Warray-bounds when it appears that it should not (initial report). Attached is a small reproducer, which has been derived from arch/powerpc/kernel/signal_32.c.

$ clang -fsyntax-only -Weverything warray-bounds.c
warray-bounds.c:12:27: warning: array index 3 is past the end of the array (which contains 1 element) [-Warray-bounds]
                __asm__ goto("" : : "r"(oldset->sig[3] >> 32) : : failed);
                                        ^           ~
warray-bounds.c:4:2: note: array 'sig' declared here
        unsigned long sig[NSIG_WORDS];
        ^
warray-bounds.c:16:27: warning: array index 2 is past the end of the array (which contains 1 element) [-Warray-bounds]
                __asm__ goto("" : : "r"(oldset->sig[2] >> 32) : : failed);
                                        ^           ~
warray-bounds.c:4:2: note: array 'sig' declared here
        unsigned long sig[NSIG_WORDS];
        ^
warray-bounds.c:20:27: warning: array index 1 is past the end of the array (which contains 1 element) [-Warray-bounds]
                __asm__ goto("" : : "r"(oldset->sig[1] >> 32) : : failed);
                                        ^           ~
warray-bounds.c:4:2: note: array 'sig' declared here
        unsigned long sig[NSIG_WORDS];
        ^
3 warnings generated.

$ gcc -fsyntax-only -Wall -Wextra -Wpedantic warray-bounds.c

This appears similar to #35463 but the second function appears to do the right thing and not warn so I do not think that it is the same situation.

[efriedma-quic]

It looks like something is going wrong with CFG analysis for asm goto, and we're treating the operand of the asm goto as "reachable" regardless of the surrounding CFG. The warning should be suppressed.

Briefly looked at it, but it's a bit more complicated than I thought. If someone wants to look, maybe start with AnalysisBasedWarnings::IssueWarnings and CFGBuilder::VisitGCCAsmStmt?

[nickdesaulniers]

https://reviews.llvm.org/D116059