Why implement this in the front end rather than at the gimple level? Jason
On Tue, May 10, 2016 at 2:19 PM, Marek Polacek <pola...@redhat.com> wrote: > Over the years, we got several PRs that suggested to add a warning that would > warn about unreachable statements between `switch (cond)' and the first case. > In some cases our -Wuninitialized warning can detect such a case, but mostly > not. This patch is an attempt to add a proper warning about this peculiar > case. I chose to not warn about declarations between switch and the first > case, because we use that in the codebase and I think this kind of use is > fine. As it's usually the case, some obscure cases cropped up during testing, > this time those were Duff's devices, so I had to go the extra mile to handle > them specially. > > This is a C FE part only; I'd like to hear some feedback first before I plunge > into the C++ FE. > > Bootstrapped/regtested on x86_64-linux, ok for trunk? > > 2016-05-10 Marek Polacek <pola...@redhat.com> > > PR c/49859 > * c.opt (Wswitch-unreachable): New option. > > * c-parser.c: Include tree-iterator.h. > (c_parser_switch_statement): Implement -Wswitch-unreachable warning. > > * doc/invoke.texi: Document -Wswitch-unreachable. > > * gcc.dg/Wjump-misses-init-1.c: Use -Wno-switch-unreachable. > * gcc.dg/nested-func-1.c: Likewise. > * gcc.dg/pr67784-4.c: Likewise. > * gcc.dg/Wswitch-unreachable-1.c: New test. > * gcc.dg/c99-vla-jump-5.c (f): Add dg-warning. > * gcc.dg/switch-warn-1.c (foo1): Likewise. > * c-c++-common/goacc/sb-2.c: Likewise. > * gcc.dg/gomp/block-10.c: Likewise. > * gcc.dg/gomp/block-9.c: Likewise. > * gcc.dg/gomp/target-1.c: Likewise. > * gcc.dg/gomp/target-2.c: Likewise. > * gcc.dg/gomp/taskgroup-1.c: Likewise. > * gcc.dg/gomp/teams-1.c: Likewise. > > diff --git gcc/c-family/c.opt gcc/c-family/c.opt > index bdc6ee0..8b6fdbb 100644 > --- gcc/c-family/c.opt > +++ gcc/c-family/c.opt > @@ -634,6 +634,11 @@ Wswitch-bool > C ObjC C++ ObjC++ Var(warn_switch_bool) Warning Init(1) > Warn about switches with boolean controlling expression. > > +Wswitch-unreachable > +C ObjC C++ ObjC++ Var(warn_switch_unreachable) Warning Init(1) > +Warn about statements between switch's controlling expression and the first > +case. > + > Wtemplates > C++ ObjC++ Var(warn_templates) Warning > Warn on primary template declaration. > diff --git gcc/c/c-parser.c gcc/c/c-parser.c > index 6523c08..bdf8e8e 100644 > --- gcc/c/c-parser.c > +++ gcc/c/c-parser.c > @@ -58,6 +58,7 @@ along with GCC; see the file COPYING3. If not see > #include "c-family/c-indentation.h" > #include "gimple-expr.h" > #include "context.h" > +#include "tree-iterator.h" > > /* We need to walk over decls with incomplete struct/union/enum types > after parsing the whole translation unit. > @@ -5605,7 +5606,30 @@ c_parser_switch_statement (c_parser *parser, bool > *if_p) > c_start_case (switch_loc, switch_cond_loc, expr, explicit_cast_p); > save_break = c_break_label; > c_break_label = NULL_TREE; > + location_t first_loc = (c_parser_next_token_is (parser, CPP_OPEN_BRACE) > + ? c_parser_peek_2nd_token (parser)->location > + : c_parser_peek_token (parser)->location); > body = c_parser_c99_block_statement (parser, if_p); > + tree first = expr_first (TREE_CODE (body) == BIND_EXPR > + ? BIND_EXPR_BODY (body) : body); > + /* Statements between `switch' and the first case are never executed. */ > + if (warn_switch_unreachable > + && first != NULL_TREE > + && TREE_CODE (first) != CASE_LABEL_EXPR > + && TREE_CODE (first) != LABEL_EXPR) > + { > + if (TREE_CODE (first) == DECL_EXPR > + && DECL_INITIAL (DECL_EXPR_DECL (first)) == NULL_TREE) > + /* Don't warn for a declaration, but warn for an initialization. */; > + else if (TREE_CODE (first) == GOTO_EXPR > + && TREE_CODE (GOTO_DESTINATION (first)) == LABEL_DECL > + && DECL_ARTIFICIAL (GOTO_DESTINATION (first))) > + /* Don't warn for compiler-generated gotos. These occur in Duff's > + devices, for example. */; > + else > + warning_at (first_loc, OPT_Wswitch_unreachable, > + "statement will never be executed"); > + } > c_finish_case (body, ce.original_type); > if (c_break_label) > { > diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi > index a54a0af..8f9c186 100644 > --- gcc/doc/invoke.texi > +++ gcc/doc/invoke.texi > @@ -297,7 +297,8 @@ Objective-C and Objective-C++ Dialects}. > -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{]} @gol > -Wsuggest-final-types @gol -Wsuggest-final-methods -Wsuggest-override @gol > -Wmissing-format-attribute -Wsubobject-linkage @gol > --Wswitch -Wswitch-default -Wswitch-enum -Wswitch-bool -Wsync-nand @gol > +-Wswitch -Wswitch-default -Wswitch-enum -Wswitch-bool @gol > +-Wswitch-unreachable -Wsync-nand @gol > -Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs @gol > -Wtype-limits -Wundef @gol > -Wuninitialized -Wunknown-pragmas -Wunsafe-loop-optimizations @gol > @@ -4135,6 +4136,39 @@ switch ((int) (a == 4)) > @end smallexample > This warning is enabled by default for C and C++ programs. > > +@item -Wswitch-unreachable > +@opindex Wswitch-unreachable > +@opindex Wno-switch-unreachable > +Warn whenever a @code{switch} statement contains statements between the > +controlling expression and the first case label, which will never be > +executed. For example: > +@smallexample > +@group > +switch (cond) > + @{ > + i = 15; > + @dots{} > + case 5: > + @dots{} > + @} > +@end group > +@end smallexample > +@option{-Wswitch-unreachable} will not warn if the statement between the > +controlling expression and the first case label is just a declaration: > +@smallexample > +@group > +switch (cond) > + @{ > + int i; > + @dots{} > + case 5: > + i = 5; > + @dots{} > + @} > +@end group > +@end smallexample > +This warning is enabled by default for C and C++ programs. > + > @item -Wsync-nand @r{(C and C++ only)} > @opindex Wsync-nand > @opindex Wno-sync-nand > diff --git gcc/testsuite/c-c++-common/goacc/sb-2.c > gcc/testsuite/c-c++-common/goacc/sb-2.c > index a6760ec..e986af3 100644 > --- gcc/testsuite/c-c++-common/goacc/sb-2.c > +++ gcc/testsuite/c-c++-common/goacc/sb-2.c > @@ -4,19 +4,19 @@ void foo(int i) > { > switch (i) // { dg-error "invalid entry to OpenACC structured block" } > { > - #pragma acc parallel > + #pragma acc parallel // { dg-warning "statement will never be executed" } > { case 0:; } > } > > switch (i) // { dg-error "invalid entry to OpenACC structured block" } > { > - #pragma acc kernels > + #pragma acc kernels // { dg-warning "statement will never be executed" } > { case 0:; } > } > > switch (i) // { dg-error "invalid entry to OpenACC structured block" } > { > - #pragma acc data > + #pragma acc data // { dg-warning "statement will never be executed" } > { case 0:; } > } > } > diff --git gcc/testsuite/gcc.dg/Wjump-misses-init-1.c > gcc/testsuite/gcc.dg/Wjump-misses-init-1.c > index 86117f1..71809be 100644 > --- gcc/testsuite/gcc.dg/Wjump-misses-init-1.c > +++ gcc/testsuite/gcc.dg/Wjump-misses-init-1.c > @@ -1,5 +1,5 @@ > /* { dg-do compile } */ > -/* { dg-options "-Wjump-misses-init" } */ > +/* { dg-options "-Wjump-misses-init -Wno-switch-unreachable" } */ > int > f1 (int a) > { > diff --git gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c > gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c > index e69de29..ec620d3 100644 > --- gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c > +++ gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c > @@ -0,0 +1,135 @@ > +/* PR c/49859 */ > +/* { dg-do compile } */ > +/* { dg-options "-Wswitch-unreachable" } */ > + > +extern void foo (int); > +extern int j; > + > +void > +fn0 (int i) > +{ > + switch (i) > + { > + int k; > + case 1: > + k = 11; > + foo (k); > + } > + > + switch (i) > + j = 10; /* { dg-warning "statement will never be executed" } */ > + > + switch (i) > + ; > + > + switch (i) > + { > + int t = 10; /* { dg-warning "statement will never be executed" } */ > + default: > + foo (t); > + } > + > + switch (i) > + { > + j = 12; /* { dg-warning "statement will never be executed" } */ > + default: > + foo (j); > + } > + > + int o; > + switch (i) > + { > + o = 333; /* { dg-warning "statement will never be executed" } */ > + case 4: break; > + default: > + foo (o); > + } > + > + switch (i) > + switch (j) /* { dg-warning "statement will never be executed" } */ > + { > + o = 42; /* { dg-warning "statement will never be executed" } */ > + case 8:; > + } > + > + switch (i) > + { > + int l = 3; /* { dg-warning "statement will never be executed" } */ > + o = 5; > + j = 7; > + ++l; > + } > + > + switch (i) > + if (j != 3) /* { dg-warning "statement will never be executed" } */ > + foo (j); > + > + switch (i) > + while (1) > + foo (0); > + > + switch (i) > + while (i > 5) { } > + > + switch (i) > + goto X; /* { dg-warning "statement will never be executed" } */ > +X: > + > + switch (i) > + do > + foo (1); > + while (1); > + > + switch (i) > + for (;;) > + foo (-1); > + > + switch (i) > + default: > + j = 6; > + > + switch (i) > + default: > + j = sizeof (struct { int i; }); > + > + switch (i) > + { > + typedef int T; > + case 3: > + { > + T x = 5; > + foo (x); > + } > + } > + > + switch (i) > + { > + static int g; > + default: > + foo (g); > + } > + > + switch (i) > + { > +L: > + j = 16; > + default: > + if (j < 5) > + goto L; > + break; > + } > + > + switch (i) > + { > + int A[i]; /* { dg-warning "statement will never be executed" } */ > + default: /* { dg-error "switch jumps into scope" } */ > + break; > + } > + > + switch (i) > + { > + int A[3]; > + default: > + break; > + } > +} > diff --git gcc/testsuite/gcc.dg/c99-vla-jump-5.c > gcc/testsuite/gcc.dg/c99-vla-jump-5.c > index fc5e04d..b5b85fd 100644 > --- gcc/testsuite/gcc.dg/c99-vla-jump-5.c > +++ gcc/testsuite/gcc.dg/c99-vla-jump-5.c > @@ -15,7 +15,7 @@ void > f (int a, int b) > { > switch (a) { > - int v[b]; > + int v[b]; /* { dg-warning "statement will never be executed" } */ > case 2: /* { dg-error "switch jumps into scope of identifier with variably > modified type" } */ > default: /* { dg-error "switch jumps into scope of identifier with > variably modified type" } */ > switch (a) > diff --git gcc/testsuite/gcc.dg/gomp/block-10.c > gcc/testsuite/gcc.dg/gomp/block-10.c > index 69ae3c0..29c2d91 100644 > --- gcc/testsuite/gcc.dg/gomp/block-10.c > +++ gcc/testsuite/gcc.dg/gomp/block-10.c > @@ -5,28 +5,28 @@ void foo(int i) > int j; > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp parallel > + #pragma omp parallel // { dg-warning "statement will never be executed" } > { case 0:; } > } > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp for > + #pragma omp for // { dg-warning "statement will never be executed" } > for (j = 0; j < 10; ++ j) > { case 1:; } > } > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp critical > + #pragma omp critical // { dg-warning "statement will never be executed" } > { case 2:; } > } > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp master > + #pragma omp master // { dg-warning "statement will never be executed" } > { case 3:; } > } > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp sections > + #pragma omp sections // { dg-warning "statement will never be executed" } > { case 4:; > #pragma omp section > { case 5:; } > @@ -34,7 +34,7 @@ void foo(int i) > } > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp ordered > + #pragma omp ordered // { dg-warning "statement will never be executed" } > { default:; } > } > } > diff --git gcc/testsuite/gcc.dg/gomp/block-9.c > gcc/testsuite/gcc.dg/gomp/block-9.c > index 2fae3de..202599f 100644 > --- gcc/testsuite/gcc.dg/gomp/block-9.c > +++ gcc/testsuite/gcc.dg/gomp/block-9.c > @@ -5,7 +5,7 @@ void foo(int i) > int j; > switch (i) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp parallel > + #pragma omp parallel // { dg-warning "statement will never be executed" } > { case 0:; } > #pragma omp for > for (j = 0; j < 10; ++ j) > diff --git gcc/testsuite/gcc.dg/gomp/target-1.c > gcc/testsuite/gcc.dg/gomp/target-1.c > index aaa6a14..6bc5eb9 100644 > --- gcc/testsuite/gcc.dg/gomp/target-1.c > +++ gcc/testsuite/gcc.dg/gomp/target-1.c > @@ -23,7 +23,7 @@ foo (int x) > > switch (x) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp target > + #pragma omp target // { dg-warning "statement will never be executed" } > { case 0:; } > } > } > diff --git gcc/testsuite/gcc.dg/gomp/target-2.c > gcc/testsuite/gcc.dg/gomp/target-2.c > index 3a7afc4..c5c38d8 100644 > --- gcc/testsuite/gcc.dg/gomp/target-2.c > +++ gcc/testsuite/gcc.dg/gomp/target-2.c > @@ -23,7 +23,7 @@ foo (int x, int y) > > switch (x) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp target data map(tofrom: y) > + #pragma omp target data map(tofrom: y) // { dg-warning "statement will > never be executed" } > { case 0:; } > } > } > diff --git gcc/testsuite/gcc.dg/gomp/taskgroup-1.c > gcc/testsuite/gcc.dg/gomp/taskgroup-1.c > index 1997e0c..f39b7ef 100644 > --- gcc/testsuite/gcc.dg/gomp/taskgroup-1.c > +++ gcc/testsuite/gcc.dg/gomp/taskgroup-1.c > @@ -23,7 +23,7 @@ foo (int x) > > switch (x) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp taskgroup > + #pragma omp taskgroup // { dg-warning "statement will never be executed" } > { case 0:; } > } > } > diff --git gcc/testsuite/gcc.dg/gomp/teams-1.c > gcc/testsuite/gcc.dg/gomp/teams-1.c > index ad5b100..db7f50b 100644 > --- gcc/testsuite/gcc.dg/gomp/teams-1.c > +++ gcc/testsuite/gcc.dg/gomp/teams-1.c > @@ -23,7 +23,7 @@ foo (int x) > > switch (x) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp target teams > + #pragma omp target teams // { dg-warning "statement will never be > executed" } > { case 0:; } > } > } > @@ -54,7 +54,7 @@ bar (int x) > > switch (x) // { dg-error "invalid entry to OpenMP structured block" } > { > - #pragma omp target > + #pragma omp target // { dg-warning "statement will never be executed" } > #pragma omp teams > { case 0:; } > } > diff --git gcc/testsuite/gcc.dg/nested-func-1.c > gcc/testsuite/gcc.dg/nested-func-1.c > index cb26e89..2052a6f 100644 > --- gcc/testsuite/gcc.dg/nested-func-1.c > +++ gcc/testsuite/gcc.dg/nested-func-1.c > @@ -1,7 +1,7 @@ > /* Test for proper errors for break and continue in nested functions. */ > /* Origin: Joseph Myers <j...@polyomino.org.uk> */ > /* { dg-do compile } */ > -/* { dg-options "" } */ > +/* { dg-options "-Wno-switch-unreachable" } */ > > void > foo (int a) > diff --git gcc/testsuite/gcc.dg/pr67784-4.c gcc/testsuite/gcc.dg/pr67784-4.c > index 81a43fd..5462080 100644 > --- gcc/testsuite/gcc.dg/pr67784-4.c > +++ gcc/testsuite/gcc.dg/pr67784-4.c > @@ -1,6 +1,6 @@ > /* PR c/67784 */ > /* { dg-do compile } */ > -/* { dg-options "" } */ > +/* { dg-options "-Wno-switch-unreachable" } */ > > typedef int T; > > diff --git gcc/testsuite/gcc.dg/switch-warn-1.c > gcc/testsuite/gcc.dg/switch-warn-1.c > index 04ca4e3..58fbd7d 100644 > --- gcc/testsuite/gcc.dg/switch-warn-1.c > +++ gcc/testsuite/gcc.dg/switch-warn-1.c > @@ -11,7 +11,7 @@ foo1 (unsigned char i) > { > switch (i) > { > - case -1: /* { dg-warning "case label value is less than minimum value > for type" } */ > + case -1: /* { dg-warning "case label value is less than minimum value > for type|statement will never be executed" } */ > return 1; > case 256: /* { dg-warning "case label value exceeds maximum value for > type" } */ > return 2; > > Marek