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+STR in Thumb 32 decode problem
+
+Hi 
+
+It seems that QEMU does not have a proper check on the STR instruction in Thumb32 mode.
+
+Specifically, the machine code is 0xf84f0ddd, which is 0b1111 1000 0100 1111 0000 1101 1101 1101. 
+This is an STR (immediate, Thumb) instruction with a T4 encoding scheme.
+
+The symbols is 
+
+Rn = 1111
+Rt = 0000
+P = 1
+U = 0
+W = 1
+
+The decode ASL is below:
+
+if P == ‘1’ && U == ‘1’ && W == ‘0’ then SEE STRT;
+if Rn == ‘1101’ && P == ‘1’ && U == ‘0’ && W == ‘1’ && imm8 == ‘00000100’ then SEE PUSH;
+if Rn == ‘1111’ || (P == ‘0’ && W == ‘0’) then UNDEFINED;
+t = UInt(Rt); n = UInt(Rn); imm32 = ZeroExtend(imm8, 32);
+index = (P == ‘1’); add = (U == ‘1’); wback = (W == ‘1’);
+if t == 15 || (wback && n == t) then UNPREDICTABLE;
+
+When Rn == 1111, it should be an undefined instruction, which should raise SEGILL signal. However, it seems that QEMU does not check this constraint, which should be a bug. Many thanks
+
+Regards
+Muhui
+
+NB that for Arm mode the equivalent case is UNPREDICTABLE only in the writeback case (and has defined behaviour for non-writeback), so we need to enforce the UNDEF on rn==15 for Thumb decode only.
+
+
+Patch sent to list: if you could test it against whatever your test case was that would be helpful.
+https://<email address hidden>/
+
+PS: out of interest, why/how were you checking should-UNDEF cases ?
+
+
+We just test the patched version. It looks well. Now QEMU would raise SEGILL signals, which should be the right behavior.
+
+We are not checking should-UNDEF cases in particular. This is a case we observed and checked manually when doing a research project with QEMU. 
+
+Patch has been merged:
+https://gitlab.com/qemu-project/qemu/-/commit/8196fe9d83d6519128b5
+