[BuildLibCalls] Add globalmemonly to math libcalls that only modify errno

[MugilanGN]

Summary

Certain math libcalls, such as acos and exp, only modify the global variable errno. This automatically adds the globalmemonly attribute to those calls. One use case is in removing unnecessary loads of the call arguments, since the calls will have no side-effects on them.

[nikic]

Is it safe to assume that errno is a "global value" in the LLVM sense? For example, here's how the musl implementation looks like: https://github.com/ifduyue/musl/blob/master/src/errno/__errno_location.c I believe C only requires that errno expands to a thread-local modifiable lvalue, but it does not necessarily have to correspond to a global.

[jdoerfert]

In D109648#2996192, @nikic wrote:

Is it safe to assume that errno is a "global value" in the LLVM sense? For example, here's how the musl implementation looks like: https://github.com/ifduyue/musl/blob/master/src/errno/__errno_location.c I believe C only requires that errno expands to a thread-local modifiable lvalue, but it does not necessarily have to correspond to a global.

Can we use a target hook to determine if it is? (errno is not the only use case but a good one).

llvm/lib/Transforms/Utils/BuildLibCalls.cpp

 
 STATISTIC(NumReadNone, "Number of functions inferred as readnone");
 STATISTIC(NumInaccessibleMemOnly,
           "Number of functions inferred as inaccessiblememonly");
 STATISTIC(NumReadOnly, "Number of functions inferred as readonly");
 STATISTIC(NumArgMemOnly, "Number of functions inferred as argmemonly");
 STATISTIC(NumInaccessibleMemOrArgMemOnly,
           "Number of functions inferred as inaccessiblemem_or_argmemonly");
+STATISTIC(NumGlobalMemOnly, "Number of functions inferred as globalmemonly");
 STATISTIC(NumNoUnwind, "Number of functions inferred as nounwind");
 STATISTIC(NumNoCapture, "Number of arguments inferred as nocapture");
 STATISTIC(NumWriteOnlyArg, "Number of arguments inferred as writeonly");
 STATISTIC(NumSExtArg, "Number of arguments inferred as signext");
 STATISTIC(NumReadOnlyArg, "Number of arguments inferred as readonly");
 STATISTIC(NumNoAlias, "Number of function returns inferred as noalias");
 STATISTIC(NumNoUndef, "Number of function returns inferred as noundef returns");
 STATISTIC(NumReturnedArg, "Number of arguments inferred as returned");

 static bool setOnlyAccessesInaccessibleMemOrArgMem(Function &F) {
   if (F.onlyAccessesInaccessibleMemOrArgMem())
     return false;
   F.setOnlyAccessesInaccessibleMemOrArgMem();
   ++NumInaccessibleMemOrArgMemOnly;
   return true;
 }
 
+static bool setOnlyAccessesGlobalMemory(Function &F) {
+  if (F.onlyAccessesGlobalMemory())
+    return false;
+  F.setOnlyAccessesGlobalMemory();
+  ++NumGlobalMemOnly;
+  return true;
+}
+
 static bool setDoesNotThrow(Function &F) {
   if (F.doesNotThrow())
     return false;
   F.setDoesNotThrow();
   ++NumNoUnwind;
   return true;
 }
 
bool llvm::inferLibFuncAttributes(Function &F, const TargetLibraryInfo &TLI) {
   case LibFunc_ldexp:
   case LibFunc_ldexpf:
   case LibFunc_ldexpl:
     Changed |= setSignExtendedArg(F, 1);
     Changed |= setWillReturn(F);
     return Changed;
   case LibFunc_abs:
   case LibFunc_acos:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_acosf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_acosh:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_acoshf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_acoshl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_acosl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_asin:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_asinf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_asinh:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_asinhf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_asinhl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_asinl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atan:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atan2:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atan2f:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atan2l:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atanf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atanh:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atanhf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atanhl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_atanl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_cbrt:
   case LibFunc_cbrtf:
   case LibFunc_cbrtl:
   case LibFunc_ceil:
   case LibFunc_ceilf:
   case LibFunc_ceill:
   case LibFunc_copysign:
   case LibFunc_copysignf:
   case LibFunc_copysignl:
   case LibFunc_cos:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_cosh:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_coshf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_coshl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_cosf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_cosl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_cospi:
   case LibFunc_cospif:
   case LibFunc_exp:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_expf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_expl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_exp2:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_exp2f:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_exp2l:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_expm1:
   case LibFunc_expm1f:
   case LibFunc_expm1l:
   case LibFunc_fabs:
   case LibFunc_fabsf:
   case LibFunc_fabsl:
   case LibFunc_ffs:
   case LibFunc_ffsl:
   case LibFunc_ffsll:
   case LibFunc_floor:
   case LibFunc_floorf:
   case LibFunc_floorl:
   case LibFunc_fls:
   case LibFunc_flsl:
   case LibFunc_flsll:
   case LibFunc_fmax:
   case LibFunc_fmaxf:
   case LibFunc_fmaxl:
   case LibFunc_fmin:
   case LibFunc_fminf:
   case LibFunc_fminl:
   case LibFunc_fmod:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_fmodf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_fmodl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_isascii:
   case LibFunc_isdigit:
   case LibFunc_labs:
   case LibFunc_llabs:
   case LibFunc_log:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log10:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log10f:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log10l:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log1p:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log1pf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log1pl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log2:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log2f:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_log2l:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_logb:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_logbf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_logbl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_logf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_logl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_nearbyint:
   case LibFunc_nearbyintf:
   case LibFunc_nearbyintl:
   case LibFunc_pow:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_powf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_powl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_rint:
   case LibFunc_rintf:
   case LibFunc_rintl:
   case LibFunc_round:
   case LibFunc_roundf:
   case LibFunc_roundl:
   case LibFunc_sin:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sincospif_stret:
   case LibFunc_sinf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sinh:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sinhf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sinhl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sinl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sinpi:
   case LibFunc_sinpif:
   case LibFunc_sqrt:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sqrtf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_sqrtl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_strnlen:
   case LibFunc_tan:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_tanf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_tanh:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_tanhf:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_tanhl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_tanl:
+    Changed |= setOnlyAccessesGlobalMemory(F);
+    return Changed;
   case LibFunc_toascii:
   case LibFunc_trunc:
   case LibFunc_truncf:
   case LibFunc_truncl:
     Changed |= setDoesNotThrow(F);
     Changed |= setDoesNotFreeMemory(F);
     Changed |= setWillReturn(F);
     return Changed;