[PATCH] D43695: [IPSCCP] mark musttail calls overdefined (PR36485)

[Fedor Indutny via Phabricator via llvm-commits]

indutny created this revision.
indutny added reviewers: davide, sanjoy, bruno, dberlin, llvm-commits.

`musttail` call must be followed by a `ret` instruction with a return
value of the call as the operand. During constant propagation, the
return value maybe folded directly into `ret` instruction, breaking the
`musttail` semantics. Mark `musttail` result as overdefined to prevent
this.


Repository:
  rL LLVM

https://reviews.llvm.org/D43695

Files:
  lib/Transforms/Scalar/SCCP.cpp
  test/Transforms/SCCP/musttail-call.ll


Index: test/Transforms/SCCP/musttail-call.ll
===================================================================
--- /dev/null
+++ test/Transforms/SCCP/musttail-call.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -ipsccp -S | FileCheck %s
+
+declare i32 @external()
+
+define i8* @start(i8* %p, i8* %endp, i32 %match) {
+  %i0 = icmp ne i8* %p, %endp
+  br i1 %i0, label %b0_has_data, label %b1_no_data
+b0_has_data:
+  %i2 = load i8, i8* %p
+  switch i8 %i2, label %b2_switch_otherwise [ i8 32, label %b3_case_32 ]
+b1_no_data:
+  %i3 = bitcast i8* (i8*, i8*, i32)* @start to i8*
+  ret i8* %i3
+b2_switch_otherwise:
+  %i4 = musttail call i8* @start(i8* %p, i8* %endp, i32 0)
+  ret i8* %i4
+b3_case_32:
+  %i5 = musttail call i8* @invoke(i8* %p, i8* %endp, i32 0)
+  ret i8* %i5
+}
+
+define internal i8* @invoke(i8* %p, i8* %endp, i32 %match) {
+  %i1 = call i32 @external()
+  %i2 = musttail call i8* @start(i8* %p, i8* %endp, i32 0)
+  ret i8* %i2
+}
+
Index: lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- lib/Transforms/Scalar/SCCP.cpp
+++ lib/Transforms/Scalar/SCCP.cpp
@@ -1229,6 +1229,10 @@
     // If so, propagate the return value of the callee into this call result.
     mergeInValue(I, TFRVI->second);
   }
+
+  // Replacing `musttail` instructions with constant breaks `musttail` semantics
+  if (CS.isMustTailCall())
+    return markOverdefined(I);
 }
 
 void SCCPSolver::Solve() {


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