Category 0: Warnings where LLVM/clang explicitly warns that it does not know of the given (GCC) warning option.

 Index | GCC warning message & GCC warning option                                                                              | LLVM/clang warning message & LLVM/clang warning option
 ===========================================================================================================================================================================================================================
   0   | warning: traditional C lacks a separate namespace for labels, identifier '<id>' conflicts [-Wtraditional]             | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   1   | warning: '<type>' switch expression not converted to '<type>' in ISO C [-Wtraditional]                                | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   2   | warning: traditional C rejects ISO C style function definitions [-Wtraditional]                                       | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   3   | warning: passing argument <num> of '<id>' as integer rather than floating due to prototype [-Wtraditional-conversion] | warning: unknown warning option '-Wtraditional-conversion'; did you mean '-Wliteral-conversion'? [-Wunknown-warning-option]
       | warning: passing argument <num> of '<builtin-function>' as signed due to prototype [-Wtraditional-conversion]         | 
       | warning: passing argument <num> of '<id>' as floating rather than complex due to prototype [-Wtraditional-conversion] | 
       | warning: passing argument <num> of '<id>' as complex rather than integer due to prototype [-Wtraditional-conversion]  | 
       | warning: passing argument <num> of '<id>' as unsigned due to prototype [-Wtraditional-conversion]                     | 
       | warning: passing argument <num> of '<id>' as complex rather than floating due to prototype [-Wtraditional-conversion] | 
       | warning: passing argument <num> of '<id>' as signed due to prototype [-Wtraditional-conversion]                       | 
       | warning: passing argument <num> of '<id>' as integer rather than complex due to prototype [-Wtraditional-conversion]  | 
       | warning: passing argument <num> of '<builtin-function>' as unsigned due to prototype [-Wtraditional-conversion]       | 
       | warning: passing argument <num> of '<id>' as floating rather than integer due to prototype [-Wtraditional-conversion] | 
   4   | warning: stack usage might be <num> bytes [-Wstack-usage=]                                                            | warning: unknown warning option '-Wstack-usage=<num>'; did you mean '-Wtype-safety'? [-Wunknown-warning-option]
   5   | warning: switch jumps over variable initialization [-Wjump-misses-init]                                               | warning: unknown warning option '-Wjump-misses-init' [-Wunknown-warning-option]
   6   | warning: '<storage-class-specifier>' is not at beginning of declaration [-Wold-style-declaration]                     | warning: unknown warning option '-Wold-style-declaration'; did you mean '-Wout-of-line-declaration'? [-Wunknown-warning-option]
   7   | warning: passing argument <num> of '<id>' with different width due to prototype [-Wtraditional-conversion]            | warning: unknown warning option '-Wtraditional-conversion'; did you mean '-Wliteral-conversion'? [-Wunknown-warning-option]
   8   | warning: ISO C99 does not support '<feature>' [-Wc99-c11-compat]                                                      | warning: unknown warning option '-Wc99-c11-compat' [-Wunknown-warning-option]
   9   | warning: stack usage is <num> bytes [-Wstack-usage=]                                                                  | warning: unknown warning option '-Wstack-usage=<num>'; did you mean '-Wtype-safety'? [-Wunknown-warning-option]
   10  | warning: ISO C90 doesn't support unnamed structs/unions [-Wc99-c11-compat]                                            | warning: unknown warning option '-Wc99-c11-compat' [-Wunknown-warning-option]
       | warning: ISO C99 doesn't support unnamed structs/unions [-Wc99-c11-compat]                                            | warning: unknown warning option '-Wc99-c11-compat' [-Wunknown-warning-option]
   11  | warning: stack usage might be unbounded [-Wstack-usage=]                                                              | warning: unknown warning option '-Wstack-usage=<num>'; did you mean '-Wtype-safety'? [-Wunknown-warning-option]
   12  | warning: non-static declaration of '<id>' follows <storage-class-specifier> declaration [-Wtraditional]               | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   13  | warning: traditional C rejects automatic aggregate initialization [-Wtraditional]                                     | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   14  | warning: the meaning of '<id>' is different in traditional C [-Wtraditional]                                          | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   15  | warning: traditional C rejects the "<suffix>" suffix [-Wtraditional]                                                  | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   16  | warning: traditional C rejects initialization of unions [-Wtraditional]                                               | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   17  | warning: unsuffixed float constant [-Wunsuffixed-float-constants]                                                     | warning: unknown warning option '-Wunsuffixed-float-constants' [-Wunknown-warning-option]
   18  | warning: traditional C rejects string constant concatenation [-Wtraditional]                                          | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   19  | warning: traditional C rejects the unary plus operator [-Wtraditional]                                                | warning: unknown warning option '-Wtraditional' [-Wunknown-warning-option]
   20  | warning: jump skips variable initialization [-Wjump-misses-init]                                                      | warning: unknown warning option '-Wjump-misses-init' [-Wunknown-warning-option]
   21  | warning: function might be candidate for attribute '<attribute>' [-Wsuggest-attribute=<attribute>]                    | warning: unknown warning option '-Wsuggest-attribute=<attribute>' [-Wunknown-warning-option]
   22  | warning: trampoline generated for nested function '<id>' [-Wtrampolines]                                              | warning: unknown warning option '-Wtrampolines' [-Wunknown-warning-option]

 Category 1: LLVM/clang did not issue warning that GCC did.

 Index | GCC warning message & GCC warning option
 ===========================================================================================================================================================================================================================
   0   | warning: old-style function definition [-Wold-style-definition]
   1   | warning: redeclaration of '<id>' with different <attribute-name> (old visibility preserved)
   2   | warning: '<id>' is narrower than values of its type
   3   | warning: ISO C prohibits argument conversion to union type [-Wpedantic]
   4   | warning: typedef '<id>' declared '<specifier>'
   5   | warning: argument to 'sizeof' in '<builtin-function>' call is the same pointer type '<type> {aka <type>}' as the second source; expected '<type> {aka <type>}' or an explicit length [-Wsizeof-pointer-memaccess]
   6   | warning: index value is out of bound [-Warray-bounds]
   7   | warning: '<attribute>' attribute ignored on types [-Wattributes]
   8   | warning: argument to 'sizeof' in '<builtin-function>' call is the same expression as the first source; did you mean to provide an explicit length? [-Wsizeof-pointer-memaccess]
   9   | warning: duplicate declaration of '<id>' is invalid in C++ [-Wc++-compat]
   10  | warning: union defined in struct or union is not visible in C++ [-Wc++-compat]
   11  | warning: function '<id>' can never be inlined because it uses setjmp [-Winline]
   12  | warning: function '<id>' can never be inlined because it uses alloca (override using the always_inline attribute) [-Winline]
   13  | warning: invalid third argument to '<builtin-function>'; using zero
   14  | warning: argument to 'sizeof' in '<builtin-function>' call is the same pointer type '<type>' as the second source; expected '<type>' or an explicit length [-Wsizeof-pointer-memaccess]
   15  | warning: '<Objective-C format specifier>' is only allowed in Objective-C dialects [-Wformat=]
   16  | warning: assuming signed overflow does not occur when simplifying comparison of absolute value and zero [-Wstrict-overflow]
   17  | warning: '_Atomic'-qualified return type of 'main' [-Wmain]
   18  | warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
   19  | warning: assuming signed overflow does not occur when assuming that (X + c) <op> X is always true [-Wstrict-overflow]
   20  | warning: redundant redeclaration of '<id>' [-Wredundant-decls]
   21  | warning: nested extern declaration of '<id>' [-Wnested-externs]
   22  | warning: iteration <iteration-id> invokes undefined behavior [-Waggressive-loop-optimizations]
   23  | warning: '<parameter>' function might not be inlinable [-Wattributes]
   24  | warning: decrement of enumeration value is invalid in C++ [-Wc++-compat]
   25  | warning: argument to 'sizeof' in '<builtin-function>' call is the same expression as the first source; did you mean to remove the addressof? [-Wsizeof-pointer-memaccess]
   26  | warning: pointer target types incompatible in C++ [-Wc++-compat]
   27  | warning: argument to 'sizeof' in '<builtin-function>' call is the same expression as the second source; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
   28  | warning: assignment of read-only location '<string-constant>[<num>]'
   29  | warning: suggest braces around empty body in an 'else' statement [-Wempty-body]
   30  | warning: '<attribute>' attribute ignored due to conflict with '<attribute>' attribute [-Wattributes]
   31  | warning: variably modified '<id>' at file scope
   32  | warning: argument to 'sizeof' in '<builtin-function>' call is the same pointer type '<type> {aka <type>}' as the second source; expected '<type>' or an explicit length [-Wsizeof-pointer-memaccess]
   33  | warning: type-punning to incomplete type might break strict-aliasing rules [-Wstrict-aliasing]
   34  | warning: suggest braces around empty body in an 'if' statement [-Wempty-body]
   35  | warning: inline function '<function>' declared weak [-Wattributes]
   36  | warning: assuming signed overflow does not occur when changing X +- C1 cmp C2 to X cmp C2 -+ C1 [-Wstrict-overflow]
   37  | warning: inlining failed in call to '<id>': function not inlinable [-Winline]
   38  | warning: argument to 'sizeof' in '<builtin-function>' call is the same pointer type '<type>' as the first source; expected '<type>' or an explicit length [-Wsizeof-pointer-memaccess]
   39  | warning: traditional C rejects initialization of unions [-Wtraditional]
   40  | warning: increment of read-only location '<string-constant>[<num>]'
   41  | warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow]
   42  | warning: suggest braces around empty body in 'do' statement [-Wempty-body]
   43  | warning: ISO C does not support '<id>' predefined identifier [-Wpedantic]
   44  | warning: decrement of read-only location '<string-constant>[<num>]'
   45  | warning: call to <builtin-function> might overflow destination buffer
   46  | warning: invalid second argument to '<builtin-function>'; using zero
   47  | warning: argument to 'sizeof' in '<builtin-function>' call is the same pointer type '<type> {aka <type>}' as the first source; expected '<type>' or an explicit length [-Wsizeof-pointer-memaccess]
   48  | warning: argument to 'sizeof' in '<builtin-function>' call is the same expression as the first source; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
   49  | warning: the omitted middle operand in ?: will always be 'true', suggest explicit middle operand [-Wparentheses]
   50  | warning: assuming signed overflow does not occur when simplifying division [-Wstrict-overflow]
   51  | warning: declaration of '<function>' with attribute '<attribute>' follows declaration with attribute '<attribute>' [-Wattributes]
   52  | warning: size of '<id>' is <num> bytes [-Wlarger-than=]
   53  | warning: '<builtin-function>' used with constant zero length parameter; this could be due to transposed parameters [-Wmemset-transposed-args]
   54  | warning: inline function '<id>' declared but never defined
   55  | warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
   56  | warning: argument to 'sizeof' in '<builtin-function>' call is the same expression as the second source; did you mean to provide an explicit length? [-Wsizeof-pointer-memaccess]
   57  | warning: assuming pointer wraparound does not occur when comparing P +- C1 with P +- C2 [-Wstrict-overflow]
   58  | warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   59  | warning: -fassociative-math disabled; other options take precedence
   60  | warning: function declaration isn't a prototype [-Wstrict-prototypes]
   61  | warning: argument to 'sizeof' in '<builtin-function>' call is the same expression as the second source; did you mean to remove the addressof? [-Wsizeof-pointer-memaccess]
   62  | warning: inlining failed in call to '<id>': function body not available [-Winline]
   63  | warning: assuming signed overflow does not occur when determining that expression is always non-negative [-Wstrict-overflow]
   64  | warning: assuming signed overflow does not occur when combining constants around a comparison [-Wstrict-overflow]
   65  | warning: argument to 'sizeof' in '<builtin-function>' call is the same pointer type '<type> {aka <type>}' as the first source; expected '<type> {aka <type>}' or an explicit length [-Wsizeof-pointer-memaccess]
   66  | warning: conflicting types for built-in function 'dremf'
   67  | warning: attempt to free a non-heap object '<id>' [-Wfree-nonheap-object]

 Note: Generated using GCC 5.3.0 and LLVM/clang 3.8.0 (early Jan 2016 trunk) using the DejaGNU suites: gcc/testsuite/gcc.dg and gcc/testsuite/c-c++-common