Never tick primitive string literalswip/libdw-prof

This is a more aggressive approach to the problem initially solved in
f5b275a239d2554c4da0b7621211642bf3b10650, where top-level primitive string
literals were being wrapped by ticks. This breaks the Core invariant descirbed
in Note [CoreSyn top-level string literals]. However, the previous approach was
incomplete and left several places where inappropriate ticks could sneak in.

This commit kills the problem at the source: we simply never tick any primitive
string literal expression. The assumption here is that these expressions are
destined for the top-level, where they cannot be ticked, anyways. So even if
they haven't been floated out yet there is no reason to tick them.

This partially reverts commit f5b275a239d2554c4da0b7621211642bf3b10650.

Test Plan: Validate with `-g`

Reviewers: austin, scpmw, simonpj, simonmar, dfeuer

Subscribers: dfeuer, simonmar, thomie

Differential Revision: https://phabricator.haskell.org/D3063

1 files changed, 5 insertions, 0 deletions

diff --git a/compiler/coreSyn/CoreUtils.hs b/compiler/coreSyn/CoreUtils.hs
index 3d5f4bcb5a..ca67e24ca3 100644
--- a/compiler/coreSyn/CoreUtils.hs
+++ b/compiler/coreSyn/CoreUtils.hs
@@ -305,6 +305,11 @@ mkTick t orig_expr = mkTick' id id orig_expr
           -> CoreExpr
   mkTick' top rest expr = case expr of
 
+    -- Never tick primitive string literals. These should ultimately float up to
+    -- the top-level where they must be unadorned. See Note
+    -- [CoreSyn top-level string literals] for details.
+    _ | exprIsLiteralString expr          -> expr
+
     -- Cost centre ticks should never be reordered relative to each
     -- other. Therefore we can stop whenever two collide.
     Tick t2 e