[Bug target/93934] Unnecessary fld of uninitialized float stack variable results in ub of valid C++ code

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93934

--- Comment #18 from Uroš Bizjak <ubizjak at gmail dot com> ---
The following patch fixes the PR, see the comment inline:

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diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
index 6e2b7920d2b..b87490fe544 100644
--- a/gcc/config/i386/i386-expand.c
+++ b/gcc/config/i386/i386-expand.c
@@ -4094,6 +4094,15 @@ ix86_expand_fp_movcc (rtx operands[])
          && !TARGET_64BIT))
     return false;

+  /* Disable SFmode and DFmode x87 FCMOV with trapping math
+     to prevent speculative load using FLDL/FLDS from uninitialized
+     memory location, which can contain sNaN value.  FLDL/FLDS traps
+     on sNaN, see PR93934.  */
+
+  if ((mode == SFmode || mode == DFmode)
+      && flag_trapping_math)
+    return false;
+
   /* The floating point conditional move instructions don't directly
      support conditions resulting from a signed integer comparison.  */

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