[ARM] [Assembler] Support negative immediates for A32, T32 and T16
Summary:
To support negative immediates for certain arithmetic instructions, the
instruction is converted to the inverse instruction with a negated (or inverted)
immediate. For example, "ADD r0, r1, #FFFFFFFF" cannot be encoded as an ADD
instruction. However, "SUB r0, r1, #1" is equivalent.
These conversions are different from instruction aliases. An alias maps
several assembler instructions onto one encoding. A conversion, however, maps
an *invalid* instruction--e.g. with an immediate that cannot be represented in
the encoding--to a different (but equivalent) instruction.
Several instructions with negative immediates were being converted already, but
this was not systematically tested, nor did it cover all instructions.
This patch implements all possible substitutions for ARM, Thumb1 and
Thumb2 assembler and adds tests. It also adds a feature flag
(-mattr=+no-neg-immediates) to turn these substitutions off. This is
helpful for users who want their code to assemble to exactly what they
wrote.
Reviewers: t.p.northover, rovka, samparker, javed.absar, peter.smith, rengolin
Reviewed By: javed.absar
Subscribers: aadg, aemerson, llvm-commits
Differential Revision: https://reviews.llvm.org/D30571
llvm-svn: 298380
diff --git a/llvm/lib/Target/ARM/ARMSubtarget.h b/llvm/lib/Target/ARM/ARMSubtarget.h
index ae67293..46c3857 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.h
+++ b/llvm/lib/Target/ARM/ARMSubtarget.h
@@ -351,6 +351,10 @@
/// UseSjLjEH - If true, the target uses SjLj exception handling (e.g. iOS).
bool UseSjLjEH = false;
+ /// Implicitly convert an instruction to a different one if its immediates
+ /// cannot be encoded. For example, ADD r0, r1, #FFFFFFFF -> SUB r0, r1, #1.
+ bool NegativeImmediates = true;
+
/// stackAlignment - The minimum alignment known to hold of the stack frame on
/// entry to the function and which must be maintained by every function.
unsigned stackAlignment = 4;