[llvm-commits] proposed patch: shift when shift amount >= bit width

[Lauro Ramos Venancio]

The attached patch unify the handle of shifts when the shift amount is
greater or equal the bit width.

Nowadays, LLVM evaluates shl(i32 X,32) to 0 in some places and to
undef in other places.
For compatibility purpose, this patch implements the gcc behavior.

This fix the miscompilation of the file ffmpeg/libavformat/mpeg.c.

Proposed behavior:
=== shl(X,N) =====
if (N<bitwidth(X))   result = X << N
if (N>=bitwidth(X))   result = 0;

=== Lshr(X,N) =====
if (N<bitwidth(X))   result = X >> N
if (N>=bitwidth(X))   result = 0;

=== Ashr(X,N) =====
if (N<bitwidth(X))   result = X >> N
if (N>=bitwidth(X) && X<0)   result = -1;
if (N>=bitwidth(X) && X>=0)   result = 0;


Lauro
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