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authorRichard Henderson <rth@twiddle.net>2014-03-18 08:21:44 -0700
committerRichard Henderson <rth@twiddle.net>2014-04-18 16:57:36 -0700
commit20022fa15f6a8ddc24a8f9d7d177312fecc7fb3a (patch)
tree23c354810892c26a639ad0ef95f5011ce1685fa0 /tcg
parentad5171dbd419ffa9e10de766e1c3198f2ae62dfa (diff)
tcg: Use "unspecified behavior" for shifts
Change the definition such that shifts are not allowed to crash for any input. Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Richard Henderson <rth@twiddle.net>
Diffstat (limited to 'tcg')
-rw-r--r--tcg/README18
1 files changed, 13 insertions, 5 deletions
diff --git a/tcg/README b/tcg/README
index f1782123b7..776e9259e3 100644
--- a/tcg/README
+++ b/tcg/README
@@ -36,6 +36,12 @@ or a memory location which is stored in a register outside QEMU TBs
A TCG "basic block" corresponds to a list of instructions terminated
by a branch instruction.
+An operation with "undefined behavior" may result in a crash.
+
+An operation with "unspecified behavior" shall not crash. However,
+the result may be one of several possibilities so may be considered
+an "undefined result".
+
3) Intermediate representation
3.1) Introduction
@@ -239,23 +245,25 @@ t0=t1|~t2
* shl_i32/i64 t0, t1, t2
-t0=t1 << t2. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+t0=t1 << t2. Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* shr_i32/i64 t0, t1, t2
-t0=t1 >> t2 (unsigned). Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+t0=t1 >> t2 (unsigned). Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* sar_i32/i64 t0, t1, t2
-t0=t1 >> t2 (signed). Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+t0=t1 >> t2 (signed). Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* rotl_i32/i64 t0, t1, t2
-Rotation of t2 bits to the left. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+Rotation of t2 bits to the left.
+Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* rotr_i32/i64 t0, t1, t2
-Rotation of t2 bits to the right. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+Rotation of t2 bits to the right.
+Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
********* Misc