[PATCH] D48134: [CodeGen] make nan builtins pure rather than const (PR37778)

[Sanjay Patel via Phabricator via cfe-commits]

spatel created this revision.
spatel added reviewers: gfalcon, lebedev.ri.
Herald added a subscriber: mcrosier.

https://bugs.llvm.org/show_bug.cgi?id=37778
...shows a miscompile resulting from marking nan builtins as 'const'.

The nan libcalls/builtins take a pointer argument:
http://www.cplusplus.com/reference/cmath/nan-function/
...and the chars dereferenced by that arg are used to fill in the NaN constant payload bits.

"const" means that the pointer argument isn't dereferenced. That's translated to "readnone" in LLVM.
"pure" means that the pointer argument may be dereferenced. That's translated to "readonly" in LLVM.

This change prevents the IR optimizer from killing the lead-up to the nan call here:

  double a() {
    char buf[4];
    buf[0] = buf[1] = buf[2] = '9';
    buf[3] = '\0';
    return __builtin_nan(buf);
  }

...the optimizer isn't currently able to simplify this to a constant as we might hope, but this patch should solve the miscompile.


https://reviews.llvm.org/D48134

Files:
  include/clang/Basic/Builtins.def
  test/CodeGen/math-builtins.c


Index: test/CodeGen/math-builtins.c
===================================================================
--- test/CodeGen/math-builtins.c
+++ test/CodeGen/math-builtins.c
@@ -90,25 +90,25 @@
 
   __builtin_nan(c);        __builtin_nanf(c);       __builtin_nanl(c); __builtin_nanf128(c);
 
-// NO__ERRNO: declare double @nan(i8*) [[READNONE]]
-// NO__ERRNO: declare float @nanf(i8*) [[READNONE]]
-// NO__ERRNO: declare x86_fp80 @nanl(i8*) [[READNONE]]
-// NO__ERRNO: declare fp128 @nanf128(i8*) [[READNONE]]
-// HAS_ERRNO: declare double @nan(i8*) [[READNONE:#[0-9]+]]
-// HAS_ERRNO: declare float @nanf(i8*) [[READNONE]]
-// HAS_ERRNO: declare x86_fp80 @nanl(i8*) [[READNONE]]
-// HAS_ERRNO: declare fp128 @nanf128(i8*) [[READNONE]]
+// NO__ERRNO: declare double @nan(i8*) [[PURE:#[0-9]+]]
+// NO__ERRNO: declare float @nanf(i8*) [[PURE]]
+// NO__ERRNO: declare x86_fp80 @nanl(i8*) [[PURE]]
+// NO__ERRNO: declare fp128 @nanf128(i8*) [[PURE]]
+// HAS_ERRNO: declare double @nan(i8*) [[PURE:#[0-9]+]]
+// HAS_ERRNO: declare float @nanf(i8*) [[PURE]]
+// HAS_ERRNO: declare x86_fp80 @nanl(i8*) [[PURE]]
+// HAS_ERRNO: declare fp128 @nanf128(i8*) [[PURE]]
 
   __builtin_nans(c);        __builtin_nansf(c);       __builtin_nansl(c); __builtin_nansf128(c);
 
-// NO__ERRNO: declare double @nans(i8*) [[READNONE]]
-// NO__ERRNO: declare float @nansf(i8*) [[READNONE]]
-// NO__ERRNO: declare x86_fp80 @nansl(i8*) [[READNONE]]
-// NO__ERRNO: declare fp128 @nansf128(i8*) [[READNONE]]
-// HAS_ERRNO: declare double @nans(i8*) [[READNONE]]
-// HAS_ERRNO: declare float @nansf(i8*) [[READNONE]]
-// HAS_ERRNO: declare x86_fp80 @nansl(i8*) [[READNONE]]
-// HAS_ERRNO: declare fp128 @nansf128(i8*) [[READNONE]]
+// NO__ERRNO: declare double @nans(i8*) [[PURE]]
+// NO__ERRNO: declare float @nansf(i8*) [[PURE]]
+// NO__ERRNO: declare x86_fp80 @nansl(i8*) [[PURE]]
+// NO__ERRNO: declare fp128 @nansf128(i8*) [[PURE]]
+// HAS_ERRNO: declare double @nans(i8*) [[PURE]]
+// HAS_ERRNO: declare float @nansf(i8*) [[PURE]]
+// HAS_ERRNO: declare x86_fp80 @nansl(i8*) [[PURE]]
+// HAS_ERRNO: declare fp128 @nansf128(i8*) [[PURE]]
 
   __builtin_pow(f,f);        __builtin_powf(f,f);       __builtin_powl(f,f);
 
@@ -188,7 +188,7 @@
 // NO__ERRNO: declare double @cbrt(double) [[READNONE]]
 // NO__ERRNO: declare float @cbrtf(float) [[READNONE]]
 // NO__ERRNO: declare x86_fp80 @cbrtl(x86_fp80) [[READNONE]]
-// HAS_ERRNO: declare double @cbrt(double) [[READNONE]]
+// HAS_ERRNO: declare double @cbrt(double) [[READNONE:#[0-9]+]]
 // HAS_ERRNO: declare float @cbrtf(float) [[READNONE]]
 // HAS_ERRNO: declare x86_fp80 @cbrtl(x86_fp80) [[READNONE]]
 
@@ -581,9 +581,11 @@
 // NO__ERRNO: attributes [[READNONE]] = { {{.*}}readnone{{.*}} }
 // NO__ERRNO: attributes [[READNONE_INTRINSIC]] = { {{.*}}readnone{{.*}} }
 // NO__ERRNO: attributes [[NOT_READNONE]] = { nounwind "correctly{{.*}} }
+// NO__ERRNO: attributes [[PURE]] = { {{.*}}readonly{{.*}} }
 
 // HAS_ERRNO: attributes [[NOT_READNONE]] = { nounwind "correctly{{.*}} }
 // HAS_ERRNO: attributes [[READNONE_INTRINSIC]] = { {{.*}}readnone{{.*}} }
+// HAS_ERRNO: attributes [[PURE]] = { {{.*}}readonly{{.*}} }
 // HAS_ERRNO: attributes [[READNONE]] = { {{.*}}readnone{{.*}} }
 
 // HAS_ERRNO_GNU: attributes [[READNONE_INTRINSIC]] = { {{.*}}readnone{{.*}} }
Index: include/clang/Basic/Builtins.def
===================================================================
--- include/clang/Basic/Builtins.def
+++ include/clang/Basic/Builtins.def
@@ -137,14 +137,14 @@
 BUILTIN(__builtin_modf , "ddd*"  , "Fn")
 BUILTIN(__builtin_modff, "fff*"  , "Fn")
 BUILTIN(__builtin_modfl, "LdLdLd*", "Fn")
-BUILTIN(__builtin_nan,  "dcC*" , "ncF")
-BUILTIN(__builtin_nanf, "fcC*" , "ncF")
-BUILTIN(__builtin_nanl, "LdcC*", "ncF")
-BUILTIN(__builtin_nanf128, "LLdcC*", "ncF")
-BUILTIN(__builtin_nans,  "dcC*" , "ncF")
-BUILTIN(__builtin_nansf, "fcC*" , "ncF")
-BUILTIN(__builtin_nansl, "LdcC*", "ncF")
-BUILTIN(__builtin_nansf128, "LLdcC*", "ncF")
+BUILTIN(__builtin_nan,  "dcC*" , "FnU")
+BUILTIN(__builtin_nanf, "fcC*" , "FnU")
+BUILTIN(__builtin_nanl, "LdcC*", "FnU")
+BUILTIN(__builtin_nanf128, "LLdcC*", "FnU")
+BUILTIN(__builtin_nans,  "dcC*" , "FnU")
+BUILTIN(__builtin_nansf, "fcC*" , "FnU")
+BUILTIN(__builtin_nansl, "LdcC*", "FnU")
+BUILTIN(__builtin_nansf128, "LLdcC*", "FnU")
 BUILTIN(__builtin_powi , "ddi"  , "Fnc")
 BUILTIN(__builtin_powif, "ffi"  , "Fnc")
 BUILTIN(__builtin_powil, "LdLdi", "Fnc")


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