[PATCH] D66439: [LibFunc] "free" captures the pointer operand

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert created this revision.
jdoerfert added reviewers: sanjoy, arsenm, hfinkel.
Herald added subscribers: bollu, hiraditya, wdng.
Herald added a project: LLVM.
jdoerfert added reviewers: reames, fhahn.

We used to mark the free operand as "nocapture", though, arguably, there
is no reason "free" could not put the pointer value in a "free-table",
thereby effectively "capturing" it.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D66439

Files:
  llvm/lib/Transforms/Utils/BuildLibCalls.cpp
  llvm/test/Transforms/InferFunctionAttrs/annotate.ll


Index: llvm/test/Transforms/InferFunctionAttrs/annotate.ll
===================================================================
--- llvm/test/Transforms/InferFunctionAttrs/annotate.ll
+++ llvm/test/Transforms/InferFunctionAttrs/annotate.ll
@@ -451,7 +451,8 @@
 ; CHECK: declare i64 @fread(i8* nocapture, i64, i64, %opaque* nocapture) [[G1]]
 declare i64 @fread(i8*, i64, i64, %opaque*)
 
-; CHECK: declare void @free(i8* nocapture) [[G3:#[0-9]+]]
+; Make sure there is *no* nocapture here.
+; CHECK: declare void @free(i8*) [[G3:#[0-9]+]]
 declare void @free(i8*)
 
 ; CHECK: declare double @frexp(double, i32* nocapture) [[G1]]
Index: llvm/lib/Transforms/Utils/BuildLibCalls.cpp
===================================================================
--- llvm/lib/Transforms/Utils/BuildLibCalls.cpp
+++ llvm/lib/Transforms/Utils/BuildLibCalls.cpp
@@ -429,7 +429,6 @@
     Changed |= setOnlyReadsMemory(F, 1);
     return Changed;
   case LibFunc_feof:
-  case LibFunc_free:
   case LibFunc_fseek:
   case LibFunc_ftell:
   case LibFunc_fgetc:
@@ -446,6 +445,10 @@
     Changed |= setDoesNotThrow(F);
     Changed |= setDoesNotCapture(F, 0);
     return Changed;
+  case LibFunc_free:
+    // Free is nothrow but does capture the argument.
+    Changed |= setDoesNotThrow(F);
+    return Changed;
   case LibFunc_ferror:
     Changed |= setDoesNotThrow(F);
     Changed |= setDoesNotCapture(F, 0);


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